Implement nonlinear core correction

[antoine-levitt]

https://aip.scitation.org/doi/10.1063/1.4793260

[mfherbst]

And here is the data: https://github.com/cp2k/cp2k/blob/master/data/NLCC_POTENTIALS Shouldn't be too hard to do.

[antoine-levitt]

The important equations are (A5) and (A6). It's just adding an atomic-centered gaussian density to the density used in the XC term. Trivial to do, and should be OK with our terms interface.

[antoine-levitt]

Seems the NLCC is only useful for some elements though

[azadoks]

Also discussed in #759 .

[mfherbst]

This is now done for UPF files and whenever PseudoPotentialIO provides NLCC.