Add support for computing adsorption energies

[WardLT]

Add a recipe which combines a molecule and surface, which involves both periodic boundary conditions and recipes which involve a molecule plus something else.

To do list:

• Switch to using extxyz for communicating molecular structures (supports fixed atoms and PBC)
• Build a data structure to describe a surface
• Store molecule + surface structures in the MoleculeRecord
• Create a recipe to generate surfaces
• Test it for a few known cases