Adding tests of computed medium epsilon vs expected

[momchil-flex]

I ported some tests that compare expected epsilon from various medium initializations. This revealed one issue with the conductivity, in that EPSILON_0 was missing. Perhaps there was some logic to that change, and we can revert to the original file (and change the conductivity in the test to 0.1 / EPSILON_0), but then we have to change the docstrings, because the conductivity will no longer be in units of S/micron.

The docstrings are correct w.r.t. the current change, but they are a bit confusing. When we say

        Defined such that the imaginary part of the complex permittivity at angular frequency omega
        is given by conductivity/omega.

We don't mean the relative complex permittivity; we mean the one in SI units. So the imaginary part of the relative permittivity is given by conductivity / omega / EPSILON_0.