Simplify the API related to the coordinate transformations about satellitetoolbox.jl

[ronisbr]

Hi @benelsen

You are right, thanks for the suggestion. I think I will leave the API simplified until we have the IAU2000B. The first implementation, as suggested by Vallado, will be using CIO approach. In this case, the selection of the model will be clear based on the frames. In the future, we can do what you proposed and then add a keyword to switch from FK5 to IAU2000B.

As for the automate fetching, it is very easy to do. I will see if I can do (if you want to help, PRs are welcome!!).

[benelsen]

Leaving the option to specify a model and only defaulting to one using the specified frames might have been interesting if there was a need to implement truncated models of nutation (IAU2000B) for performance reasons or revisions to the model. (maybe even automate fetching the eops, or generalize to other bodies).