Modify applied electric field Kernels

[mangerij]

Adjust applied electric field Kernels to accept

E_total = -\nabla \Phi + E_appl (k,\omega)

where \Phi comes from the quasi-static depolarization effects.

This will allow pulsed fields that propagate through the sample. In principle E_appl (k, \omega) can be decomposed further to E_appl (k, \omega) + E_rad (k, \omega) where E_rad is a self-radiating electric field from solving Maxwell's equations for a given source term dP/dt (further work)