The coverage_category states are returned instead of their number.
Updated tests to work with new function signature.
Added optional splitter_category parameter.

Partial support for (optional) splitting of mapping file prior to filtering.
Added some preliminary error handling unit tests and pulled the "meaty" code
into a "private" function ```__filter_sample_ids_from_category_state_coverage()```.
Finished implementation of splitter_category.

Still need to add more unit tests.
Added unit test for splitter_category.

Also cleaned up some duplicate code in the unit tests and added docstrings to
new helper function.
Updated ChangeLog.
Updated splitter_category docstring.

Improves clarity.
Merge branch 'master' into timeseries_filtering

Conflicts:
	ChangeLog.md
Merge pull request #630 from jrrideout/timeseries_filtering

Further improvements to timeseries filtering.