Refactor initial groups implementation into BaseGroups objects
More refactor progress to put everything into groups attr
Move groups tests into seperate module
Put in workaround for issue #1471
Add __repr__ for TableGroups and ColumnGroups
Add column group_by
Change test_groups to use plain functions not class
Add tests of copying and slicing grouped tables
Support item slicing and mutable operations on grouped tables
Fix bug trying to delete nonexistent _groups attr
Support copying table groups in Table.copy()
Make copying a grouped column pass tests
Add tests of grouped item access
Move keys and group_keys from BaseGroups into TableGroups
Add tests of group keys() and values()