stubbing out feature and commenting on old @wip
adding in expected values (poss to remove)
Ignore feature tests from PEP8
Improve sub groups feature test
Rename _mongo to _mongo_driver for clarity
Refactor nested merge in database

Refactor the code that merged grouped periodic query results into the
nested data structure that is returned by backdrop.

This code simplifies the code `nested_merge` function and moves it out
into it's own module (`backdrop.core.nested_merge`).

This also adds collected fields to subgroups as well as just the root.

All parts of the function are now recursive so multiple levels of
grouping can be handled.

WIP: Some tests have not been written yet.
Add more tests for nested merge

Also fix a bug in `remove_collected_key` where default was being removed
from subgroups.
Propagate collected fields in subgroups
PEP8
Enable feature test for subgroup collect
Add some function comments to nested_merge
Merge branch 'master' into volumesInSubGroups

Conflicts:
	backdrop/read/config/development.py
Refactor periodic grouped response

Remove duplication
Factor out period specific code

<3 deleting code.
Return None for a collect that has no data

Returns null in the JSON because we need to differentiate between
non-existent data and zero data.
Remove TODO comment

We did do.
Merge pull request #146 from alphagov/volumesInSubGroups

Collected fields in sub groups