i-pi calculator

[sirmarcel]

Hey all, great work!

Together with @thorben-frank I've made a PR for i-pi that implements a SO3LR calculator:

i-pi/i-pi#388

To make this work, some things are required to be fixed:

• The mlff calculator doesn't seem to handle neighborlist invalidation properly
• i-pi's ase calculator expects stress to be returned as well. I'm not sure if it's better to write a different ase calculator on the side of i-pi or just return zeros here.