Refactoring SLSim - Create Galaxy class

[jacob-hjortlund]

Current Status

Currently the representation of a lens or source galaxy and a potential corresponding transient is spread across multiple different classes, including SourceBase, Lens, and Deflector.

Proposed Solution

Collect the responsibility of representing a galaxy and any related transient, including any necessary instantiation of composites, into a single Galaxy class. Instances of this class will be created by the Population class when samples are drawn from a Catalog, see #225 and #226 respectively. Additionally takes a callable mass_model and light_model to be instantiated from the provided galaxy parameters, see INSERT ISSUE HERE. This is part of the overall SLSim refactor, see #223.

To-Do List

• Create Galaxy skeleton code.
• Add ingestion of galaxy parameters.
• Add ingestion of mass and light profiles.
• Add ingestion / handling of provided / missing transient meta data.
• Add method for providing galaxy / transient position.
• Add method for providing galaxy / transient magnitude.
• Add method for providing galaxy mass profile.
• Add method for providing galaxy light profile.
• Add unit tests.