Twaml

- Have selection API use `pandas.DataFrame.eval` instead of `pandas.eval`.
- Require numexpr so selection is accelerated.

Make `dataset.__add__` and `dataset.append` handle the metadata

Major:
- The `selection` feature for `dataset.from_root` has been reworked to use pandas `eval` instead of the less functional numpy ufuncing. It's now more `TTree::Draw` like, see the docs for an example.

Minor:
- `dataset.from_root` can now also just take a string as a single file input, (doesn't have to be a single entry list to make a single file work)

Fixes bug in `twaml.dataset.to_pytables` which always printing warning about existing file.