Sgerberding adding python eos

[Gerbeset]

PR Summary

Added StiffGas into module.cpp so that stiff gas can be called in python bindings.

Can be called using (for example):

from singularity_eos import StiffGas
gm1 = 5./3. - 1
Cv = 2

gamma = 5./3.
q = -(gamma/(gamma-1))
P_inf = 1
sing_stiff_gas = StiffGas(gm1, Cv, P_inf, q)

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by using the make format command after configuring with cmake.
• N/A Document any new features, update documentation for changes made.
• Make sure the copyright notice on any files you modified is up to date.
• After creating a pull request, note it in the CHANGELOG.md file.
• LANL employees: make sure tests pass both on the github CI and on the Darwin CI

[jhp-lanl]

Can you add a one-line test to make sure the object can be created here:

https://github.com/lanl/singularity-eos/blob/main/test/python_bindings.py#L67

As you can see, there is no thought towards correctness of parameters, so it should be an easy addition

[jhp-lanl]

Looks good now. I'll merge when the tests pass