Adding a "weighting variable" to a Grid

[WillJRoper]

To further automate grid interpolation I've added a "weight variable" attribute to a Grid, in future grid files this will define the Synthesizer component attribute the grids are normalised by. With this known the corresponding component attribute can be automatically extracted at spectra generation.

Example usage for this:

• SPS models currently use stars.initial_masses spectra weights.
• AGN models currently use bh.bolometric_luminosity spectra weights.

Note this breaks nothing and I also fixed some old ionising luminosity nomenclature which was hovering in the depths for no reason.

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

[WillJRoper]

@christopherlovell want to chat about this this week at some point?