support for reading in xyz files

[JamesB-1qbit]

One can now use xyz files as input for SecondQuantizedMolecule

[alexfleury-sb]

I made a suggestion, using zip.

Also, would it be relevant to add a method to write XYZ? Now we don't support features changing the coordinates, but I am foreseeing that it can be a possibility. Here is a function I used in the past to write XYZ files from a pyscf molecule:

def write_xyz(mol, filename="mol.xzy"):
    """Write an xyz file for this Molecule.
    Parameters
    ----------
    mol : pyscf.gto.mol
        PySCF molecule.
    filename : str
        Filename for the xyz file.
    """

    bohr_to_ansgtrom = 0.529177
    coords = mol.atom_coords() * bohr_to_ansgtrom
    print(coords)
    print(filename)

    n_atoms = coords.shape[0]

    content = "{}\n\n".format(n_atoms)
    for i in range(n_atoms):
        element = mol.atom_pure_symbol(i)
        x = coords[i, 0]
        y = coords[i, 1]
        z = coords[i, 2]

        content += "{:2} {:10.5f} {:10.5f} {:10.5f}\n".format(element, x, y, z)

    content = content[:-1]

    with open(filename, 'w') as f:
        f.write(content)

[ValentinS4t1qbit]

A minor change