Refactoring SLSim - Create Sampler class

Current Status

LensPop uniformly samples from the deflector and source populations, constructs a Lens instance and checks if it is a valid strong lensing system. This leads to relatively low efficiency in creating strong lensing systems, which can hamper some use cases.

Proposed Solution

Extend LensPop to take a string / callable as input following the Sampler interface shown in #223 . The Sampler interface should take a Catalog from a Population instance and override the sample method of the Population. An example subclass, and expected default behaviour would be a UniformSampler that reproduces the behaviour of the Population sample method, see #225 . A useful subclass would a OpticalDepthSampler follows the approach in A. Sainz de Murieta et al. 2024, and takes a source redshift and the lens catalog, and samples deflector galaxies weighted by their optical depth, increasing the chances of a given deflector / source combination creating a valid Lens (see #228 ).

To-Do List

[ ] Decide on function or class implementation. Will Sampler require attributes?
[ ] Extend LensPop to take a sampler input, accepting a union of strings / callables, with strings referring to implemented samplers and a callable allowing for a drop-in replacement.
[ ] Extend LensPop to use the provided sampler when sampling from the lens and source populations.
[ ] Impement Sampler base if class approach is chosen / necessary.
[ ] Implement uniform sampler.
[ ] Implement optical depth based sampler.
[ ] Create / update unit tests.

LSST-strong-lensing / slsim