Uniform IO wavefunction files

[degennar]

Currently the IO routines handling the restart Wannier files (those used when restart_from_wannier_pwscf=.true.) are not the standard xml routines used to read/write all the wavefunctions files in CP. We should use the xml routines to write any wfc file.

Additionally, the files evc_occupied1.dat and evc_occupied2.dat can be eliminated (together with the restart_from_wannier_pwscf flag) and adapt the code to always use the standard files for the wfc of the variational orbitals, namely evc01.dat and evc02.dat.