Document beta Sobolev

[andrewfullard]

The beta Sobolev value is an escape chance for packets in specific lines. We should document this and refer back to the appropriate Lucy paper (2000?).

[TusharNaugain]

Hi @andrewfullard

Thank you for pointing out the need to document the beta Sobolev value. I agree that this is a crucial parameter for calculating the escape probability of packets in specific lines, particularly within the context of radiative transfer models.

To address this, I propose we add the following documentation:

Beta Sobolev Value and Escape Probability

The beta Sobolev value is a critical parameter for calculating the escape probability of photons in specific spectral lines within radiative transfer models. This concept leverages the Sobolev approximation, assuming that the thermal line width is small compared to the velocity gradient, simplifying the treatment of line transfer.

Reference: For an in-depth explanation, see Leon B. Lucy's paper "Computing Radiative Equilibria with Monte Carlo Techniques" (2000).

I hope this adequately addresses the issue. Please let me know if there are any additional details or modifications neede