point group of a cluster

[seshasaibehara]

• Migrated the code that I had for point_group_of_molecule from pycasm
• uses xtal.Occupant to represent a molecule
• Added tests for cluster_point_group
• Added a function xtal.Occupant.from_xyz_string() that reads Occupant from xyz file
• Added tests for xtal.Occupant.from_xyz_string()

[bpuchala]

Looks good. A couple changes I'd request:

• rename cluster_point_group to make_cluster_point_group to match the convention used elsewhere
• let's use the convention of a single underscore for modules and functions that are not in the public interface. Currently make_cluster_point_group is imported as libcasm.xtal.make_cluster_point_group. So then, unless you want additional methods in the public interface, this would mean renaming cluster_point_group.py as _cluster_point_group.py and adding a leading underscore to the methods other than make_cluster_point_group.

[bpuchala]

One more thing:

• Are AtomComponent properties or Occupant properties included in the cluster group symmetry analysis? (I don't think they are). If they are not, then maybe we should avoid having them input to make_cluster_point_group and use something else like atom_type: list[str] and atom_coordinate_cart: np.ndarray so it's not a question?