new linear panel model and group utils
Grouping sort method
check_index sanity
smarter input processing sorting
added between time model
reindex Grouping method
minor Grouping bug
removed fd
moved panel to regression folder
add panel tests
test lower case sorting in plm
minor testing
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.