How to add a new baryon field

If you wish to add a new BaryonField array -- for instance, to track the Oxygen fraction -- you'll need to do a few things.

1. Add it to the field_type structure
2. Define it in your problem type. (You'll probably also add a new test problem)?
3. Do something with it. (You'll probably need to add a new local operator, as well.)
4. If you need to advect a species as a color field, you will have to investigate how that works. Specifically, the means of conservation -- by fraction of by density -- as well as the inputs into the hydro solver.