WIP: low-Mach correction for HLLC solver

[BenWibking]

New version of #57.

[BenWibking]

@pgrete @forrestglines I'd like to get this merged in, so I can run my precipitator simulations with upstream AthenaPK and Parthenon. Please let me know what you need from me to do this.

[pgrete]

This is not the full implementation of the LHLLC solver, i.e., it just contains the fix for the pressure at the contact surface and not the shock fix (based on the velocity divergence), isn't it?

[BenWibking]

This is not the full implementation of the LHLLC solver, i.e., it just contains the fix for the pressure at the contact surface and not the shock fix (based on the velocity divergence), isn't it?

Yes, that's right.