Add PySCF grid levels to AtomicGridSizeDefault values

[awvwgk]

For reproducing PySCF grids in GauXC it would great to have them available as part of the AtomicGridSizeDefault enumerator.

[wavefunction91]

Thanks, @awvwgk. I'm in principle supportive of this, but the size is likely the least of the concerns here - can we get an example of us actually reproducing the grids (e.g. w/pruning)? I'm fine having the path, but I have some severe reservations about how they prune grids. It would be good to have that in the open so we can do AB...Z comparisons.

[awvwgk]

Can do, but I mainly care about the grid sizes here. Does GauXC already implement the same pruning scheme as used in PySCF or does this need to be implemented separately first?

[awvwgk]

See #198 for an implementation of the pruning schemes.

Tests in this PR comprehensively check the grid size definition for all elements against the values obtained by the pyscf.dft.gen_grid module for the angular and radial sizes. In #198 I am testing against selected cases for each of the PySCF pruning schemes, not sure whether making it comprehensive is useful (num elements x num grid sizes x pruning schemes = ~5k).