Improvement of excitonic WFs plotting implementation

Davide Sangallirequested to merge
fix-exc-wfs-plotting into develop

Further change in implementation for plotting WFs.

Compared to develop version, this one is OMP ported and faster. See also discussion here: !237 (merged)

It could still be improved (see comment in ypp/excitons/excitons_WFs.F about algorithm).

@muralidharrsvm7 can you test this as well ?

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