[MPL] VermaPruess equation
With this pull request, we add the VermaPruess porositypermeability 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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Edited by renchao.lu