BUG: Fix for MultiIndex to_excel() with index=False.

Issue #5616
Merge pull request #5659 from jmcnamara/bug_to_excel_index_false

BUG: Fix for MultiIndex to_excel() with index=False.
BUG: HDFStore improperly inferring a freq on datetimeindexes
Merge pull request #5674 from jreback/hdf_doc

BUG: HDFStore improperly inferring a freq on datetimeindexes
DOC: cookbook.rst update
BUG: properly handle a user function ingroupby that returns all scalars when
     grouping on a dataframe (GH5592) revisited
BUG: handle coercion of None for date/time like dtypes in a groupby result
Merge pull request #5675 from jreback/groupby_none

BUG: properly handle a user function ingroupby that returns all scalars  (GH5592)