Function to create atomic rings with n_atoms.

Note that basic geometry suggests that for spacing L between atoms the radius of the ring should be L / (2 * cos (pi / 2 - theta / 2))

n_atoms Integer, the number of atoms in the ring.
spacing The spacing between atoms in the ring in Angstroms.
basis The basis in which to perform the calculation.
atom_type String, the atomic symbol of the element in the ring. this defaults to 'H' for Hydrogen.
charge An integer giving the total molecular charge. Defaults to 0.
filename An optional string to give a filename for the molecule.

molecule A an instance of the MolecularData class.