Aperture module and SNR-optimal aperture

[jorgemarpa]

This PR builds upon previous PR #15 to implement an aperture module that enables aperture mask creation from the PRF model. It includes the following:

• aperture module with methods to compute metrics and apertures. It inputs a PRF model, or a list of PRF models for several sources in the scene, and the source magnitudes
• Implements 3 type of apertures:
  1. Simple: based on a flux completeness value. This is the simplest mask and only needs the target PRF.
  2. SNR-optimal: simirlarly to SPOC aperture, it optimizes the cumulative SNR as a funciton of pixels added to the aperture (sorted by highest SNR pixel). This needs the PRF ealuation for all sources in the scene and their expeced flux, in order to compute the noise in the aperture. It also enables including read and quantization noise for a more realistic noise estimation.
  3. Balanced: creates an aperture that satisfies crowding and flux fraction thresholds defined by the user.
• Computes flux fraction (FLFRCSAP) and crowding (CROWDSAP) for a given aperture.
• The PRF class has a get_aperture() method that uses the aperture module to compute apertures for the prf object.
• lkprf_aperture_example.ipynb notebook tutorial showcasing the new aperture module.
• Bump up version to 1.2.1 and update CHANGES

Closes / Supersedes #15