ENH: VASP Frequency Results Handling

Wanted to get the results of an IBRION=5 calculation, i.e. a phonon/frequency run. I propose two new functions for the VASP calculator:

• read_vib_modes to return tuples of eigenvalues and eigenvectors (can also be complex eigenvalues)
• get_vibrations to get a representation of the vibrations by displacing coordinates along the normal modes.

While writing the test I stumbled upon a remaining open file (the sorting file), so I changed that. Don't see why this should remain open.

This collides with !1274, so if depending on what is merged first a bit of work is needed. Happy to do that though. If helpful, I can also try to incorporate the changes of !1274 and write some tests.

Happy to get feedback on this.