Porting the flux-limiter prescription from r9793 to the latest MESA version

[JRGarza]

This PR ports the flux-limiter prescription from Levermore & Pomraning (1981), originally available in MESA r9793, to the latest version. It addresses the following:

• Models radiative transport in the transition from an optically thick to optically thin medium (i.e., from diffusion to free-streaming).
• Prevents photons from escaping at superluminal velocities in the optically thin regime.

The code introduces the following boolean control:
use_flux_limiting_with_dPrad_dm_form

When set to .true. and use_dPrad_dm_form_of_T_gradient_eqn = .true., it applies the flux-limiter prescription to compute the radiative pressure gradient in terms of the flux-limiter flxLambda as:
flxR = 4 * pi * r^2 * abs(dT^4/dm) / kap
flxLambda = (6 + 3*flxR) / (6 + 3*flxR + flxR^2)
Then, the expected radiative pressure gradient is updated as:
d_Prad_expected = d_Prad_expected / flxLambda

Additionally, the flux-limiter and flux ratio (flxR) values are stored in the stellar properties as:
flux_limit_lambda
flux_limit_R
These values can be saved in profile files by uncommenting the corresponding entries in profile_columns.list

[JRGarza]

Resolves #769
Related to #625

[Debraheem]

Awesome Jaime, I was about to give it a shot only to see that you've done all the leg work! Merging this into a branch for testing.

[JRGarza]

Thanks a lot, hope this is useful for more people.