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
I tend to just pass through `t` in this case.  Does `t.read()` bring it into memory?  I don't necessarily consider a numpy array to be any more valid a return value than a `tb.Table`.  Also, because `TableSymbol` is probably the leaf of your tree this operation will happen at the start of any `compute` call.  So if this does return a `np.ndarray` then none of the other implementations in this file are likely to be run, computation will shift over to the implementations in `numpy.py`.