STY: PEP-8 cleanup
ENH: Make handle_data overwritable by subclasses.
ENH: Add panel data model data handling.
ENH: Add abstract PanelModel class.
ENH: Allow type check tools to work with one arg
ENH: Use existing helper functions.
ENH: Update PanelLM to use PanelModel and data attribute.
BUG: Preserve shape of input.
TST: Use keywords in tests.
REF: Check method and effects compat earlier.
STY: twoways -> twoway.
STY: Respect DRY a bit.
STY: Line length
DOC: Add some documentation to Groupings methods
STY: Comment cleanup
STY: Revert call signature to what it was.
ENH: Do some more arg checking.