Address warnings in analytic tests from the Mie Gruneisen Us-up EOS

When running eos_analytic_unit_tests, a lot of warnings related to the Mie-Gruneisen Us-up appear regarding negative temperature and negative entropy. Actually, the negative entropy doesn't matter because the EOS doesn't satisfy the third law of thermodynamics. In particular, $T \to 0$ corresponds to $S \to -\infty$. But the negative temperature is something that should be addressed.