pw2wannier: encapsulation of compute_amn and compute_mmn for use elsewhere in QE

[JeromeCCP9]

• Improve pw2wannier by providing functions that can be called by other parts of QE for overlap and projection matrices; these functions to include wave function reading, etc, and need few arguments (and using QE's modules).
• Compare pw2wannier implementation with EPW's (which is better documented and may have so additional functionality).
• Plan: JJ to do light refactoring (April); discuss with Samuel, Junfeng, Nicola C
• Independent of using w90 v4's library.

[jaemolihm]

I would also be interested to join the discussions and implementation!