Inconsistent naming conventions of class member variables / C++ standards violations

Within the context of singularity-eos we have different conventions for naming private variables which are members of classes. For nearly all of the analytic EOSs (for which I was mostly responsible) we use a leading underscore for private member variables. In such class scope, this is allowed under the standard, but only if the next character is neither an underscore nor a capital letter. This is part of C++ standards, and while it is likely to work without issue, it's technically reserved.

I propose changing all of the private member variables to trailing underscores, which is consistent with the tabular EOS implementations (both spiner and EOSPAC) and is not standards violating.

Start by inventorying private member variables across the analytic EOS implementations and compare their naming with the tabular EOS implementations. Update the affected names to use trailing underscores, then verify that all references compile and the project tests pass.