Clean up implementation of pressure positivity

[ekeever1]

Negative calculated pressure will manifest in high Mach (and in magnetic simulations, low-beta) situations where internal energy fails to contribute significantly to E, and is Bad with a capital B for all kinds of reasons.

Imogen's hydrodynamic pressure calculator in freezeAndPtot.cu currently implements a minimum pressure test & imposition, due to the fact that truncation error means that Imogen does not preserve the positivity of pressure.

• The same needs to be implemented for the MHD pressure calculator (in the same manner).
• Both must be updated to take a parameterized minimum pressure
• This parameter must be stored somewhere in run.fluid
• It must be also passed to cudaAccretingStar to handle the minimum background values appropriately,
• which routine must in turn also be fixed to accept the parameter.

[ekeever1]

Pressure positivity is enforced for the hydrodynamic flux routines. Needs to be implemented for the MHD routines.

[ekeever1]

This has currently been disabled. It will need to be made more flexible because simply inserting energy to enforce positivity causes problems.