atom_slice

We should go through the method names on `Trajectory` and see if they're good. For example, for this kind of stuff, which splitting and joining trajectories, we have (including this PR) these four methods.

- acting along the atom axis: `atom_slice`, `stack`,
- acting along the frame axis: `slice`, `join`.


IMHO not just renamed, *replaced by* as the default inplace behavior changes, right?