Create a function for identifying sub-surface shells

[BvB93]

Currently the CAT.recipes.replace_surface can be used for identifying (subsets of) surface atoms.
Add an option to operate on subsurface shells, rather than the surface.

Proposed implementation:

• Add the nth_shell argument which accepts either an integer (i.e. the nth sub-surface shell) or a list with one or more integers (i.e. multiple subsurface shells).
• Add the action keywords (accepted values "raise", "warn", "ignore") specifying what to do when nth_shell is too large. For example, when nth_shell=10 but there are only 5 shells in total.