Feature request: continuum opacities/emissivities

[ThomasCeulemans]

We should add an option for adding continuum opacities/emissivities (frequency independent constant terms).
This requires some refactoring for solver::get_source_dtau and maybe even adding some datastructures to store these new things.

However, as I not the main user myself, I do not exactly know what to implement:

• Do they have some location/density/temperature/... dependence?
• Are these terms tabulated somewhere (eg in a paper, or preferably some database findable using astropy)?

If you are interested in seeing continuum contributions to emissivity/opacity being implemented in Magritte, please comment in this thread.

[jing-ze-ma]

I think this would make Magritte more impactful in producing synthetic images. Maybe frequency-dependent terms are better. For H- free-free opacity which dominates the cool star continuum in ALMA range, a standard formula to use is given here
https://ui.adsabs.harvard.edu/abs/1988A%26A...193..189J/abstract
which depends on density, temperature, and frequency.