Implement LSDA

[aromanro]

Probably this will not happen soon, but it's reasonably simple to implement.

[aromanro]

At this moment the code is not exposed in the UI and the implementation exists only for the non-uniform grid, but exposing in the UI and adding uniform grid are straightforward, so I'm closing this.
The results are close enough to the NIST values, even closer than I expected (due of the two coupled Schrodinger eqns, as opposed to the only one for LSD).