Real-space interpolation in response ground state adapter

When we calculate the xc-kernel in the magnetic response code, we need the electron density on the fine real-space grid. In order to make sure, that the resulting density is nonnegative with gridrefinement=2, we interpolate the density from the coarse grid onto the fine grid, instead of relying on the usual pw-mode interpolation.

With this addition, we are able to run a meaningful magnons test of the NiCl2 monolayer without having to add a background electron density.