Add the possibility to print el-ph matrix elements

Originally written by S. Ponc'e and C. Verdi based on the elphsum subroutine in 06/2018 and later modified by H. Lee in 06/2021.

Note that this commits removes more lines than adding (422 lines added and 1728 lines removed overall), thus improving readability of the code.

In case the el-ph module is fully decoupled from PH in the future, it should be moved with it but for now can be included there, I think.