Add support for self-shielding in heating/cooling rates calculation

[cphyc]

PR Summary

RAMSES uses some self-shiedling to compute the cooling/heating rates. This parameter is used in hydro/cooling_fine.f90. When self-shielding is activated, a "boost factor" is computed as boost =exp(-nH/0.01), and the effective density used to look-up the cooling tables is nH/boost.

PR Checklist

• [x] New features are documented, with docstrings and narrative docs
• [ ] Adds a test for any bugs fixed. Adds tests for new features.

[neutrinoceros]

@yt-fido test this please

[cphyc]

Let's hold this a bit more, we probably need testing before this goes in!

[neutrinoceros]

okay, converting to draft then !