ENH: added `adjust_label` to `Vibrations` init

Hi,

this one is just a small extension. I am also not sure if it is too releveant for many users, or maybe there is a better solution to it. The problem is as follows:

Running a vibrational analysis with a calculator like AIMS would overwrite the existing in- and output files, which is not a good thing. This option allows to adjust the label of the calculator for each displacements such that the calculations either have a different prefix, or directory, depending on how the calculator handles set_label.

Alternative suggestions are highly welcome!

Cheers,

Simon