Add tidal model parameters to API docs

[jo11he]

tidal model estimatable parameters are not yet included in the API docs, mainly because I don't anything about them.

My guess is they are strongly linked to the environment_setup.gravity_field_variations module, which is not yet documented.
I recommend to document this module first, including sufficiently extensive model descriptions, then add tidal parameters in parameter module and refer to model descriptions (from either environment_setup or the parameters module directly, depending on the pathway decided on in issue #27).

I suggest @DominicDirkx to have a look at this first.