Dataset Features

[kecnry]

This PR implements a new hook for dataset features (modifying model datasets after they are populated, but before dataset-scaling for pblum_mode='dataset-scaled' and before any cost function is computed).

This also starts on a more general framework to simplify defining new features. Right now it is still necessary to manually create the parameter-creation function as well as the allowed kinds (in parameters/feature.py) but shows how this can be defined in the feature class itself.

Eventually we may want to allow for creating these at runtime, but that will require additional thought on how these would be sent to a remote server and also should think about other cases where a user may want to write custom functionality outside of phoebe, and so is intentionally deferred here.

TODO:

• make sure the GPs have sufficient test coverage and pass tests here since the logic has been moved
• test coverage for the sinusoidal third light (or hide from users and instead keep just as an example for future development)