WIP: Add k_mu source terms

[AstroBarker]

This PR adds calculations for k_mu source terms (radiation/geodesics.hpp) for arbitrary metric/geometry as it was previously specialized to Minkowski. This was causing unphysical positions when running the torus problem. It now runs beyond the problem point and a longer run in in the queue. The leptoneq problem seems reasonable.

Also removes a print statement from plot_snap2d that I left from before.

TODO:

• There are redundant calculations (in Minowski all the sources are 0, in FMKS only 2 are non zero) and we are doing more integration than necessary. Performant way to specialize? Template on geometry?
• Write a simple test for particles around a Kerr black hole to say for sure that the Push is doing what we want.

[brryan]

Great work! I can provide comparison data for the particle pushes soon. A lot of this can be done in a unit test, but ultimately I think it would be good to compare particles that have been pushed through the driver, to ensure that driver stages are interacting appropriately with the particle pushes. In that case a purpose-specific pgen that creates a few initial particles and then outputs particle positions with particle IO may be the right choice.

[Yurlungur]

Nice work! As @brryan suggested, this is probably something we should be integrating more tightly with the geometry package, so that nothing here needs to be recalculated when there are symmetries. But that can be a future improvement.