merge of devel-interpolate-screening

[andrea-ferretti]

W-average for metallic screening.
The merge can be done, after the following issues are addressed:

• Some of the newly added files (rim_integrate_w.F, QP_interpolate_W.F) are huge, and a cleanup is definitely needed, I've left some comments in the headers of the files.
• One needs to decide whether the graphene-specific screening is included in the main release or not (personally I would leave it in a separate branch)
• Some refs in the test suite need to be updated (a phys-interp-screen branch should be created with the updated refs)

[daniele-varsano]

*I agree clean up is needed.
*Graphene specific I agree can be left out
*I cannot see the comments in the headers