feat: add molecules info to python

[pilatmartin]

Adds Molecules::info to python bindings.

Usage
Properties can be accessed directly:

molecules = Molecules("/path/to/file")
info = molecules.info()
print(info.total_atoms) # 29874

Or the result can be converted to a dict that can be serialized:

info.to_dict()

Example output (receptor.pdb from acc2 examples)

{
    "total_molecules": 1,
    "total_atoms": 29874,
    "atom_type_counts": [
      {
        "symbol": "N",
        "count": 2386
      },
      {
        "symbol": "C",
        "count": 9738
      },
      {
        "symbol": "O",
        "count": 2723
      },
      {
        "symbol": "H",
        "count": 14950
      },
      {
        "symbol": "S",
        "count": 77
      }
    ]
}