ENH: Make sure returned results is 2d.
TST: Fix signature for transform slices test
Merge pull request #1424 from jseabold/grouputils-apply-vs-transform

ENH: Consistent results for transform_slices
ENH: New Linear Panel Model
smarter input processing sorting
added between time model
ENH: Use reindex in Panel model
removed fd
moved panel to regression folder
add panel tests
test lower case sorting in plm
minor testing
ENH: Add panel data model data handling.
ENH: Add abstract PanelModel class.
ENH: Allow type check tools to work with one arg
ENH: Update PanelLM to use PanelModel and data attribute.
TST: Use keywords in tests.
REF: Check method and effects compat earlier.
STY: twoways -> twoway.
STY: Respect DRY a bit.