add ideal gas EOS with tabulated mean molecular weight

[BenWibking]

For many of our applications, we have an ideal gas but with a mean molecular weight that varies as a function of $(\rho, T)$. The temperature can be solved via root-finding on on a table of mean molecular weight.

We have this implemented in Quokka for our tabulated dilute plasma cooling, but it is not currently integrated with the EOS infrastructure. In the future, we should move this part of the code to Microphysics.

cc: @markkrumholz @aditivijayan

[BenWibking]

@zingale What format would be appropriate to put this table in? We currently read this table from an HDF5 file, but HDF5 would then have to become a Microphysics dependency.

Our existing code to read this table is here:
https://github.com/quokka-astro/quokka/blob/6aca86475ce95a8e9cec545e01dfae1edd52639c/src/GrackleDataReader.cpp#L203

[zingale]

I think that HDF5 is fine as an optional dependency.