AGN spectra, improved plotting, and tying up loose ends

The code is now in Beta with all the initially proposed functionality included (and then some). An overview of the included changes since v0.20-beta are listed below.

Changelog:

• load_data module introduced with increased functionality for loading cosmological simulation data (SIMBA, CAMELS, ILLUTRIS-TNG, etc.).
• Workflows refined including changes to the code listing during a PR to ensure consistent and PEP-8 compliant code.
• Installation updated to make full use of the capabilities of a pyproject.toml method.
• Particle age masks were fixed to behave correctly when generating spectra.
• Consistent use of Quantities covering the vast majority of the codebase.
• Miscellaneous improves to quick plotting methods.
• Introduced galaxy level spectra methods for aggregating component spectra and getting observed spectra.
• parametric.Stars argument simplified.
• Introduced optional warnings for particles/SFZH bins at grid boundaries for debugging.
• Introduced the ability to scale parametric galaxies to a fixed luminosity.
• __slots__ removed to avoid multiple bases have instance layout conflict error (with no meaningful effect on memory footprint).
• Corrected unit issues.
• fesc can now be given as a value per particle for particle Stars.
• Introduced a PhotometryCollection for easy unit association and backend sugar.
• Can now integrate particle spectra at a galaxy level.
• • Miscellaneous bug fixes.

Initial Pre-release

This is the initial prerelease of Synthesizer and a test of the automatic versioning done with setuptools_scm. At this time, all major functionality works but there's still some foundational work to be done:

• Multicomponent spectra need a full implementation.
• Blackhole spectra need a full implementation.
• Spectral data cubes need fleshing out with analysis methods.
• Particle and Parametric classes needed expanded documentation.
• Emission and attenuation models need refining for ease of use.

Plus other smaller issues and changes.