[MPL] Verma-Pruess equation

With this pull request, we add the Verma-Pruess porosity-permeability model:

\textbf{k} = \textbf{k}_0 \left(\frac{\phi - \phi_{\textrm{c}}}{\phi_0 - \phi_{\textrm{c}}}\right)^n,

where \textbf{k} is the permeability, \textbf{k}_0 is the initial permeability, \phi is the porosity, \phi_0 is the initial porosity, \phi_{\textrm{c}} is the critical porosity, and n is the exponent.

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