Feature/spherical harmonics

[moritzgubler]

Hi

I used Horton: documentation, conversion tool to construct the conversion coefficients from cartesian to spherical gaussian orbitals.

Using these coefficients I implemented def add_spherical_gto(self, c, alpha, l, m): in pyqint/cgf.py which adds a normalized, real spherical harmonic function. I also added a test in tests/test_cgf.py where I check if the overlap matrix of all spherical gaussian orbitals is the identity matrix which it is.

Let me know if you also like this feature or if you have any other suggestions for improving this feature.