Feature: add function to print all the available model components

[Nikhil0504]

Since there was no function to see all the available toggles that can go into the model_components dictionary, this is a PR to solve that.

Now we can do

import bagpipes.models as models

print(models.list_available_components())

And get the output as follows:

{'dust_emission': ['qpah', 'umin', 'gamma'], 'dust_attenuation': ['Calzetti', 'Cardelli', 'SMC', 'Salim', 'VW07'], 'agn': ['alphalam', 'betalam', 'f5100A', 'hanorm', 'sigma'], 'nebular': ['velshift'], 'star_formation_history': ['burst', 'constant', 'exponential', 'delayed', 'lognormal', 'dblplaw', 'iyer', 'continuity', 'custom', 'const_exp', 'iyer2019', 'psb_wild2020'], 'chemical_enrichment_history': ['delta', 'metallicity_bins', 'metallicity_bins_continuity', 'exp', 'lognorm', 'constant', 'two_step', 'psb_two_step']}

[ACCarnall]

Hi Nikhil, thanks for this, looks like a good idea, will go through and have a proper look when I get some more time.