BUG/CLN: fix iloc when positional indexer matched Int64Index keys

Also, drop takeable parameter from reindex(...) function family.
Merge pull request #6614 from immerrr/drop-reindex-takeable-param

BUG/CLN: fix iloc when positional indexer matched Int64Index key
ENH: Allow longtable in to_latex. Closes #6616.

DOC: Make note about booktabs package.

DOC: Add longtable to release notes.
BUG/API: allow TimeGrouper with other columns in a groupby (GH3794)
CLN/API: replace groupby.CustomGrouper with Grouper
         rename internally Grouper to _Grouper to avoid conflict
         TimeGrouper to now inherit from Grouper