Add a propagator for a full system of linearized cold plasma equations with a frequency perturbation.

[Monoclod]

This is the discretization of a linearized cold plasma fluid system perturbed by a source of frequency $\omega$.
The state variables are the first-order components of the oscillations in electron density, electron velocity, electric field and magnetic field.
Their oscillation-average counterparts (making up the plasma bulk) are passed as parameters of the solver.
Each variable is split into a real (cosine) and imaginary (sine) part to represent the complex quantity fully.