WIP: compute on HDF5 with PyTables

closes #361

- [ ] sort based on pytables indexes
- [ ] should sort always work? e.g., pytables only allows sorting on a single indexed column whereas numpy and pandas allow multiple columns
    - [ ] either let users know they need to index, instead of reading into memory or just always work
    - [ ] i've currently implemented this as basically `t.read() -> numpy sort` when pytables throws a type error
This would also be triggered by `Add([1, 2, 3], my_table)`.  Perhaps this logic can be kept within the `(ColumnWise, tb.Table)` implementation?