WIP: precipitator

[BenWibking]

This adds the precipitator pgen.
(New PR to avoid issues with actions running on forks.)

This PR depends on:

• Reflecting BC #79
• Low-Mach correction for HLLC solver #80
• TODO: well-balanced reconstruction (for hydrostatic equilibrium)
• TODO (maybe): allow user-defined spatial modulation of the cooling rate?

Parthenon PRs:

• WIP: Gaussian quadrature parthenon#875
• WIP: Piecewise cubic monotone interpolation parthenon#874

[BenWibking]

@par-hermes format

[BenWibking]

@pgrete In order to be able to merge this PR eventually, I need to either copy and paste the cooling code with my modifications into the pgen, or the cooling code should allow the pgen to provide a callback function for special modifications (like changing the cooling curve as a function of location in the box).

I assume the former is the preferred option. However, I still would like to avoid copying the cooling code completely. Is there a limited subset of the cooling code that I should copy into the pgen? Also, I can't hook into the task manager from the pgen, so I assume I should make it operator split in the same way I've implemented the turbulence driving source term?

[BenWibking]

@pgrete I think this has been made obsolete by the new way you've implemented histograms in Parthenon, no?