Atomic densities from Slater's rules

Slater's rules offer a simple way to compute somewhat realistic atomic electron densities. They are also used in the formation of some quadrature grids. The rules used in Molpro have been summarized in [J. Chem. Phys. 149, 044103 (2018)](https://doi.org/10.1063/1.5032175); in addition, one needs to assign formal occupations to the orbitals.