Synchrotron radiation

[phajy]

We'd like to update the synchrotron radiation calculation to be more realistic, including the full synchrotron spectrum rather than the $\delta$-function approach used in some of the literature.

For the electron distribution it would be good to have a function that returns the electron density $n_e(\gamma) d\gamma$ as a function of electron Lorentz factor $\gamma$. Initially we'll model this as a power law, or broken power law, between two limits, $\gamma_\text{low}$ and $\gamma_\text{high}$. Later we can then generalise this to arbitrary electron distributions.

Thinking further ahead for when we might want to do some radiative transfer (e.g., thinking about self-absorption and inverse-Compton scattering) it will also be useful to keep track of the energy density and/or spectral energy density.