ENH: Added NDFrame.slice_shift

added slice_shift func docs and removed freq argument
BUG: Change Panel.shift to use slice_shift #6605 #6826

TST: Make sure Panel.shift retains dtypes

DOC: removed previous doc entries for #6605

Re-add note about dropping shifted periods

DOC: added note about bug fix

don't pass on freq
Merge pull request #6974 from dalejung/panel_shift_revert

Panel shift revert
BUG: GroupBy.get_group raises ValueError when group key contains NaT