Investigate cavity benchmark failure for different thermodynamic pressure formulation

[pbauman]

The cavity benchmark is currently using the following equation to solve for the thermodynamic pressure: {{latex(P_{th} = P_0\left(\int_{\Omega}\frac{1}{T_0};dx\right)\left(\int_{\Omega}\frac{1}{T}\right)^{-1})}} This is what Becker and Braack use. And it works fine for us. However, if instead the "full" formulation is used (where there's mass flux across the boundary allowed), then the cavity benchmark doesn't converge. Figure out why.

Note that an input option needs to be added to switch between the formulations - right now I'm relying on commenting/recompiling for this (if the thermodynamic pressure needs to be computed that is).