Connect to GalSim

[jiwoncpark]

Accommodate GalSim's features as per PR #48, but include modules for deconvolution and flux rescaling.

[jiwoncpark]

Modified from the project description:

I'd like to begin implementing realistic/real galaxy profiles via galsim.RealGalaxy and lenstronomy.LensModel.Profiles.interpol (very similar to PR #48). This feature depends on several modules not yet implemented. The project board ties all the dependencies together.

For now, logging the surface brightness, position, size, catalog, and ID within catalog should be sufficient. The prior can be described by distributions on these "parameters." There should be a Baobab module to translate the SkyPy catalog into GalSim terms.

Main focus in LSST but implement this in a telescope-agnostic way, meaning that real galaxy images must be deconvolved and appropriately rescaled to a common unit surface brightness. Put notes for galaxy profiles, PSF, and flux in separate issues.