Bussi thermostat implementation

The structure is based on !1583 (closed) but some refactoring have been done.

This seems to be working as intended, temperature average to what is expected and atoms.get_total_energy() - self.transfered_energy remains constant.

@flokno This might be of interest to you.

I think this is important since the only really proper thermostat available for now is Nose-hoover, under the name "NPT".

Checklist

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Issue is resolved via "closes #XXXX" if applicable.