📚 Include other broadening partners than air for ExoMol

[minouHub]

💭 Description

This to-do follows the issue #585 partially solved with #586. The feature request is isolated here.

I implemented an error raised when a user tries to compute the spectrum of an ExoMol molecule with a diluent other than "air". See example below:

from radis import calc_spectrum
s = calc_spectrum(1900, 2100,         # cm-1
                  molecule='CO2',
                  isotope='1',
                  pressure=.001325,   # bar
                  Tgas=4000,
                  mole_fraction=0.1,
                  databank='exomol',  
                  verbose=1,
                  wstep='auto',
                  diluent = "He"    
                  )

NotImplementedError: Only air broadening is implemented in RADIS with ExoMol. Please reach out on https://github.com/radis/radis/issues

The issue can be solved with a bit of patience

• the list of broadening partners should be added as an attribute in the spectrum factory
• update set_broadening_coef here so that list of other broadening partners are loaded in df
• remove the bkgdatm parameter that is not used anymore and replace it with diluent