Create and use generic function for calculation of air density in physics

An increasing number of physics schemes (e.g. PUMAS and microp_aero) appear to be calculating their own air density using the equation of state. While nothing is inherently wrong with that, a possibly better scientific and software practice would be to create a function (that lives somewhere in `phys_utils`) that does that calculation, and which can then be used by the whatever physics scheme needs it.

Doing this would allow the actual calculating code to be in a single location, where it can be given a full comment and provenance info, and can be well unit/property-tested. It can also make it clear what exactly is being done in the calculation of the air density via the name of the function itself, instead of needing to look at the math/variables to deduce what is going on.

Start by locating phys_utils and the existing equation-of-state air-density calculations in PUMAS and microp_aero. Compare their inputs and conventions, then define the shared function and update those schemes to use it. Add unit/property tests and provenance documentation; done means the calculation is centralized and the shared behavior is covered.

Written by the indexing model from the issue text.