Add term diagnostics to evolve_pressure

[mikekryjak]

Adds a new flag diagnose_terms which saves each term in the pressure equation as a separate diagnostic.
This is the first instance of an extra diagnostics flag in the code because it introduces a lot of new variables. Is this the right approach, or shall we have a more generic diagnose_extra or something like this, which would enable a generic extra level of diagnostics?

I also improved the convention for the sources diagnostic. In this PR, there is a distinct component_source and user_source. The former is a source term from other components, e.g. reactions. The latter is the user-set source.

So, two questions:

• Should this be triggered by diagnose_terms, or maybe by a more generic diagnose_extra to act as the 2nd diagnostics level?
• Should I implement this for density and momentum equations too for consistency, or can we just start with the pressure equation for now?

todo:

• Implement pressure term diagnostics
• Add docs
• From Peter: don't add more diagnose levels now. Leave that until the number of diagnostics becomes a problem, at which point we'll have more time to think about what to do.