Explaintion:
The Wigner-Seitz construction in EPW was done by constructing a cell
centred unit cell. This is fine for electronic properties (this is what is done in wannier90).
However for phonon or electron-phonon properties, one can have issues when the cell
is tilded for example.
The proper way is to construct a set of WS vectors centred on pairs of atoms (phonons)
or atoms (el-ph).
In the matdyn code, a FT grid is constructed with weigths centred on pairs of atoms
and zeros everywhere else.
EPW now reproduced exactly the results of matdyn for the interpolated phonons at a
lower computation cost. Indeed we minimize the number of zeros by keeping the union
of values between all the cells.
In both cases this is very fast anyway but is important for el-ph properties.

A Wigner module was created to deal with this.

In addition, the subroutine cdiagh2 from PHonon/PH/rigid.f90 has been made public to
avoid code duplication in EPW