How to add a new baryon field

So you want to add a new BaryonField array, eh? Want 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. Something about color fields. I'll need to look into this, as I'm no longer familiar with how this works.

More later. It's dinner time.