Document how to modify models in common ways

[hersle]

The model extension tutorial should show better how to get started with some of the most common ways of modifying cosmological models:

• How do I make some tiny modifications to just a few equations? Can show symbolic substitution.
• How do I modify the gravity component? Can show effective parametrized $\mu(a(\tau),k)$ and $\eta(a(\tau),k)$.
• How do I modify the dark energy component? Can show $w_0 w_a$.
• How do I modify the dark matter component? Can show how to make it interact with a different species.
• How do I modify recombination physics? Can show simple Peebles H-only recombination.
• How do I add another species? Can show independent scalar field.