Introducing EmissionModels and docs rework (Using dependency injection for spectra generation)

[WillJRoper]

This PR introduces EmissionModels which simplify spectra generation by enabling the definition of tree-like templates for atomically defining complex models.

• An EmissionModel encompasses a single step in spectra generation, e.g. extraction, generation, attenuation, or combination.
• A mask can be applied during any of these steps.
• All spectra generation methods are now deprecated in favour of a generic get_spectra method which takes an EmissionModel and parses it generating the necessary spectra (injection).
• All commonly used sets of spectra are importable and modifiable but a user can construct their own by chaining together individual EmissionModels.
• AGN models are also encompassed by EmissionModels.

This PR also includes a complete rewrite of the docs.

This PR closes a myriad of open issues:

• Closes IR_Template should behave the same as all other emisison models #630
• Closes Comments about reprocessed spectra contradict code/don't make sense in get_spectra_* #627
• Closes Pacman double assigns attenuated instead of escape  #626
• Closes Integrated transmitted spectra apply escape fraction twice for particle components #621
• Closes Allow arbitrary masks #616
• Closes Method named wrong  #582
• Closes pacman should take dust_curve object not just alpha #468
• Closes Add separate f_esc parameters for escape from HII regions and unattenuated light #429
• Closes Allow reading a grid from a group in a HDF5 dataset #412 (no longer applicable)
• Closes Particle objects are missing docs. #318
• Closes IGM module needs documenting and adhering to style #562
• Closes Include quantised optional dependancies for loaders and dense baisis #652

Issue Type

• Enhancement

Checklist

• I have read the CONTRIBUTING.md -->
• I have added docstrings to all methods
• I have added sufficient comments to all lines
• I have made corresponding changes to the documentation
• My changes generate no pep8 errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[christopherlovell]

Documentation pages that require updating:

• components/stars/particle_stars.ipynb
• components/stars/stars.rst
• components/gas/particle_gas.ipynb
• components/gas/gas.rst
• components/blackholes/blackholes.rst
• components/blackholes/particle_blackholes.ipynb
• components/blackholes/parametric_blackholes.ipynb
• lines/lines.rst
• lines/lines.ipynb
• emission_models/emission_models.rst
• emission_models/premade_models.ipynb
• emission_models/dust_emission.ipynb
• emission_models/blackhole_emission_models.ipynb
• imaging/imaging.rst
• imaging/*.ipynb :)