Initial GroupedTable functionality: create table with indexes
Add basic aggregate method
Allow for numpy array input for groupby keys
Simplify _groupby by using Table methods
Add column aggregate method
Make aggregate return columns of same class as original
Allow for group_by keys to be Table
Update table meta for output of aggregating
Add tests of group_by and aggregate
Update group_by / aggregate docstrings
Remove orphaned code
Make sliced GroupTable return a Table
Fix typo in exception name
Simplify aggregation warning messages
Make Table __repr__ work for subclasses
Fix GroupedTable __getitem__ to select columns or slice
Add group / aggregate user docs
Add GroupedTable to the top-level table classes
Fix aggregate method doc refs
Add GroupedTable groups iterator