Semi-duplicate of (delta) density matrix

[bhourahine]

          Your fix seems fine, although I'm not sure if we should simultaneously store $\mathrm{P}(\mathbf{k})$ and $\Delta\mathrm{P}(\mathbf{k})$ at all. At this point they are of course still identical, but I don't see a reason why we could not store e.g. $\Delta\mathrm{P}(\mathbf{k})$ only and calculate $\mathrm{P}(\mathbf{k})$ on the fly in the linresp routines.

Originally posted by @vanderhe in #1367 (review)

[bhourahine]

Need to refactor the handing of the density matrix for linear response, as hybrids modify the handling of this quantity,

[vanderhe]

@bhourahine Can we change the title of this issue? ;)