ENH: Adding a NotImplementedError for simultaneous use of nrows and chunksize.

For read_csv() the user intention most likely is to get a TextFileReader, when using the chunksize option, but simultaneous use of nrows is not implemented yet. This raises now a NotImplementedError. Test and entry to current whatsnew source (v0.14.1.txt) added.
Fixes #6774
BUG: DatetimeIndex.asobject raises ValueError when contains NaT
Merge pull request #7544 from sinhrks/dtmixin

BUG: DatetimeIndex.asobject raises ValueError when contains NaT
PERF: vbench for mixed groupby with datetime (GH7555)

PERF: perform coercing and casting on groupby agg by blocks
Merge pull request #7085 from michaelaye/add_notimp_error

adding a NotImplementedError for simultaneous use of nrows and chunksize...
Merge pull request #7560 from jreback/groupby_first_perf

PERF: vbench for mixed groupby with datetime (GH7555)
API: add/clarify numpy ufunc prepare/wrap
WIP: categoricals as an internal CategoricalBlock GH5313
Categorical: add new constructor mode

Catgoricals: implement min/max/sort
CLN: integrate JanSchulz changes
     change _value_pointer back to labels

BUG: fix up PeriodIndex formatting/printing
API: Categorical sort api backwarks - now conform to Series interface (order returns a copy, sort is inplace)

ENH: delegate _reduction and ops from Series to the categorical
     to support min/max and raise TypeError on other ops (numerical) and reduction
more min/max tests for categorical as a pd.Series

Add Categorical Properties to Series

's.levels' and 's.ordered' are passed through to the underlying
Categorical instance.

Default to 'ordered' Categoricals if values are orderd

R defaults to ordered factors if the input values are ordered. Do
the same. Change the unittests accordingly.

Also reordere some unittests: catgoricals tested as is and tested as
Series/Dataframes belong to different classes.

Test that Series.ordered/.levels only works on Categorical types

Categorical: add level assignments and reordering + changed default for ordered

Add the possibility to change the levels (including longer and shorter levels).

Add a `Categorical.reorder_levels()` method. Change some naming in `Series`,
so that the methods do not clash with established standards and rename the
other categorical methods accordingly.

Do some refactorings and use a faster version in case values + levels are
passed in at creation time.

Also change the default for `ordered` to True if values + levels are passed
in at creation time.

Initial doc version for working with Categorical data

Categorical: add Categorical.mode() and use that in Series.mode()

Categorical: Rename fast_path to fastpath

Categorical: Use fastpath in some cases

Categorical: Add mode testcases for Categorical

Categorical: implement remove_unused_levels()

Both implemented in Categorical and Series

Categorical: implement value_count() for categorical series

Categorical: new examples in Categorical docstring

Categorical: Use fastpath in internal code

Categorical: check that categoricals in dataframes are also not compressed in groupby

Categorical: add `Categorical(cat)` constructor codepath and tests

Categorical: First version to fix groupby with categoricals

Categorical: make Series.astype("category") work

Categorical: Add failing testcase due to sideeffects

Categorical: NaNs handling in init

Categorical: new doc versions
remove Series accessors in favor of using .cat
added some FIXME sections to some failing tests

TST: infer correctly a multi-level index of pd.NaT and Timestamp

PERF: perf issue with indexing
Categorical: add better check for categorical in Series.cat
Categorical: doc changes
Categorical: New (failing!) unittests for slicing
API: removed need for ndim changing in Categoricals

TST: fime tests in Categorical slicing/indexing
Categorical: additional (failing) slicing testcases
Categorical: testcase: groupby based on categorical should result in categorical index
Categorical: first version of assigning testcases (failing)
BUG: fix slicing issues

ENH: add setitem to Categorical

BUG: assigning to levels not in level set now raises ValueError

BUG: side effects now tested / work correcty

BUG: require identical level set when setting

BUG: fixup groupby tests for size/count