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.