Whether to calculate dipole matrix elements, to be written in vmtxel.
This should be done when calculating WFN_fi for Bethe-Salpeter calculations
with light polarization in a finite direction. In that case, a shifted grid
WFNq_fi cannot be calculated, but we can instead use matrix elements of
$r$ in a more exact scheme. In absorption.inp, set read_vmtxel
and use_momentum. Specify the number of conduction and valence bands you will
use in BSE here with BerkeleyGW_VmtxelNumCondBands and BerkeleyGW_VmtxelNumValBands.