Class. Plot screened interaction in real space

[alexmoratalla]

Proposal to develop a function to:
read screening, make FFT and plot screening in real space

[palful]

After the latest updated on em1sdb.py, we can now compute W_GG'(q) in reciprocal space so the first step "read screening" is basically done.

We need to implement the FFT and a class to produce .xsf files.

[rreho]

Hi, in my forked branch I created a branch called 'devel-xsf' and started to write a class that reads and write xsf files. In my forked branch, I opened an issue with the tasks that have been done and a 'to-do' list. I will keep developing it also for my own research projects, but advice and comments are welcome!