Mitigation strategies for Maxwell-constructed EOS in the PTE Solvers #613

A user reported issues with Maxwell constructed EOS. The primary problem is that in a Maxwell constructed region, P = 0 for a wide range of densities and temperatures. We have a mitigation for densities, by having a minimum volume fraction step. But for temperatures, we do not, which can make the Jacobian ill conditioned. We should mitigate in two ways:

1. Adding a minimum gradient size in temperature
2. Adding a maximum temperature, to avoid temperatures diverging to NaN.