GaussianProcess docstring would benefit from enhancements

[mrtommyb]

There is an inconsistency between the celerite2 Getting Started tutorial and the Exoplanet tutorials in the use of GaussianProcess and compute. Specifically, in the celerite2 tutorial compute is called explicitly but in the Exoplanet tutorials that include celerite2, compute is implicitly called through supplying the t keyword.

I don't think this is a particular problem, but it would be good if the main GaussianProcess docstring stated that if t is supplied then the compute method is automatically run.

In addition, the theano.GaussianProcess class doesn't have a docstring so this would benefit from having the same docstring as the main GaussianProcess class, also with the addition of the note about compute.

[mrtommyb]

I would be happy to make these changes and submit a pull request, if that would be useful and you agree these changes are a good idea.

[dfm]

Sure - that would be awesome! Thanks Tom!