AGN spectra, improved plotting, and tying up loose ends

[WillJRoper]

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.

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