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
yep `t.read()` is the same as `t[:]` which reads the array into memory.

thanks for clarifying. i agree. things like `Head` and `Selection` return `ndarray`s so i couldn't decide which one was better. it's also possible to not define this at all. in fact, i'll remove it because it isn't actually doing anything