Add Bussi thermostat

[ElliottKasoar]

Now available via ASE: https://wiki.fysik.dtu.dk/ase/ase/md.html#module-ase.md.bussi, although not currently in any tagged release.

This could potentially be added similarly to #194, with an accompanying developer tutorial.

[ElliottKasoar]

One thing to note is that on setting the dynamics, this requires the initial velocities to be non-zero. This means currently this throws errors, as we don't call MaxwellBoltzmannDistribution etc. until run.

This will require changes to our general approach, as currently initialising the MD object does not change the Atoms object, but that will be necessary here.

For consistency, we may want to consider moving setting the velocities for all ensembles, but my preference would probably be to make this the exception, and preserve the current functionality otherwise.

[alinelena]

@ElliottKasoar are you working on this?

[ElliottKasoar]

@ElliottKasoar are you working on this?

I've made a start, but I'm not actively working on it right now, having realised how we handle setting the velocities is non-trivial.