[WIP] Model accessing docs

[TorkelE]

Not actually sure what to call this one, but this is the tutorial that goes through how to access stuff within a ReactionSystem.

• For a starter, it goes through basic stuff like species(rs).
• For now, I also put our main section on how accessing symbolic variables via the model (e.g. rs.X) works.
• There is also a (still working on it) bit on hierarchical models, i.e. difference between get_species and species, and rs.submodelname.X.

[TorkelE]

Superseded by #889 (could not rebase this one on master properly)