Deal with compressibility limit in new Gruneisen EOS

[jhp-lanl]

From #331 :

I think it would be good to compute a maximum density for this EOS to ensure that sound speeds aren't imaginary.

You can see we used this approach in the other Gruneisen EOS, but with a much more complicated procedure for calculating the maximum density (since it's a cubic Us-up fit).

I would recommend mirroring this approach where the user is allowed to provide a maximum density or not. If the maximum density is used, then we set it and perhaps warn the user if it's greater than the singularity in the denominator of the Hugoniot expression. If the user does not provide a maximum density, then we provide one based off of the Hugoniot fit parameter.

See this section of the other Gruneisen EOS for an example of how to do this.

singularity-eos/singularity-eos/eos/eos_gruneisen.hpp

Line 40 in f7df74f

Gruneisen(const Real C0, const Real s1, const Real s2, const Real s3, const Real G0,

Originally posted by @jhp-lanl in #331 (review)