CP2K: spin density for UKS runs (cube file, Mulliken/Hirshfeld spin per atom in the Viewer) #592

NBOMAT, via Andy 10-26: to see whether an added electron sits on the dopant or on Nb. Part of #412.
Write the spin density cube (&E_DENSITY_CUBE / spin density print) for unrestricted runs, read it as ECCE's spin density field; parse Mulliken and Hirshfeld spin populations per atom and show them in the charges panels/Viewer.