feature: enable output Wc when task=g0w0

[Fisherd99]

1. add an int parameter output_Wc_Rf_mat, 0 for no output, 1 for out lowest frequency, 2 for out all frequency
2. fix task_screened_coulomb.cpp, change conj to get_transpose(true)
3. implement Wc(q,w) -> Wc(R,w) -> W(R,t) as full atom-pair

[minyez]

Thanks. I think functionalities for writing energy_qp and Wc have been picked from other branches.

• energy_qp file can now be written by adding output_energy_qp = t in librpa.in.
• output_Wc_Rf_mat is renamed to a bool parameter output_wc_rf. Setting it to t will write by default all Wc(R,i omega) matrices under librpa.d, under name Wc_iR_<iR>_ifreq_<ifreq>.mtx. To suppress output beyond the first frequency point, add ifreq_output_wc_end = 1. Note that the whole matrix instead of atom-pair blocks will be dumped. Also, the comment string is moved from the header line to the comment session of MM format.

Could you please check both with the latest master?