The gauge vector field is used to include a uniform (but time-dependent)
external electric field in a time-dependent run for
a periodic system. An optional second row specifies the initial
value for the time derivative of the gauge field (which is set
to zero by default). By default this field is not included.
If KPointsUseSymmetries = yes, then SymmetryBreakDir
must be set in the same direction.
This is used with utility oct-dielectric_function
according to GF Bertsch, J-I Iwata, A Rubio, and K Yabana,
Phys. Rev. B62, 7998-8002 (2000).