Refactor groupby group_prod, group_var, group_mean, group_ohlc
State: closed

Followup to a previous [PR](https://github.com/pandas-dev/pandas/pull/24954).

Continue refactoring cython function implemented in tempita into fused types.

Labels:
name: Refactor
description: Internal refactoring of code
name: Groupby
name: Internals
description: Related to non-user accessible pandas implementation

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=h1) Report
> Merging [#25249](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2448e5229683acbe7de57f2d53065247aa085b1f?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25249/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #25249   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         166      166           
  Lines       52213    52213           
=======================================
  Hits        47985    47985           
  Misses       4228     4228
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.44% <ø> (ø)` | :arrow_up: |
| #single | `42.12% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=footer). Last update [2448e52...b2d93b1](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=h1) Report
> Merging [#25249](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2448e5229683acbe7de57f2d53065247aa085b1f?src=pr&el=desc) will **decrease** coverage by `0.18%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25249/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25249      +/-   ##
==========================================
- Coverage    91.9%   91.72%   -0.19%     
==========================================
  Files         166      173       +7     
  Lines       52213    52838     +625     
==========================================
+ Hits        47985    48464     +479     
- Misses       4228     4374     +146
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.28% <ø> (-0.17%)` | :arrow_down: |
| #single | `41.73% <ø> (-0.4%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `71.74% <0%> (-8.53%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `91.76% <0%> (-2%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `92.27% <0%> (-1.61%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `94.16% <0%> (-0.53%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.57% <0%> (-0.18%)` | :arrow_down: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.08% <0%> (-0.03%)` | :arrow_down: |
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.69% <0%> (ø)` | :arrow_up: |
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `84.61% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby/grouper.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGVyLnB5) | `98.16% <0%> (ø)` | :arrow_up: |
| [pandas/core/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbWlzc2luZy5weQ==) | `92.56% <0%> (ø)` | :arrow_up: |
| ... and [10 more](https://codecov.io/gh/pandas-dev/pandas/pull/25249/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=footer). Last update [2448e52...0cb991b](https://codecov.io/gh/pandas-dev/pandas/pull/25249?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).

ping on green @noamher after updated.
thanks @noamher