Document data types

[AleMorales]

At the moment we do not document the structure of objects that the user will own, which makes it harder for them to work with said objects. We should define access functions for all of them (rather than relying on dot notation) but this should documented somewhere.

Proposal:

• Always document the type of object returned by a function

• Add a docstring for the data type itself pointing at the access functions

• Document the access functions