Add extrapolation to rho = 0 for spiner tables

Right now spiner's logarithmic interpolation means that it can never extrapolate to exactly rho = 0. An easy fix would be to check if rho < the minimum density on the table, and if it is compute pressure as $P(rho, T) = \rho \Gamma(\rho_m, T)$ where here $\rho_m$ is the minimum density and $\Gamma$ is the Gruneisen coefficient. And one could do a similar thing for temperature.

Thanks @jhp-lanl for reminding me of this issue.