[T|Docu] unit test for Verma-Pruess model and documentation.

Documentation/ProjectFile/properties/property/VermaPruessModel/c_VermaPruessModel.md

+\copydoc MaterialPropertyLib::VermaPruessModel

Documentation/ProjectFile/properties/property/VermaPruessModel/t_critical_porosity.md

+\copydoc MaterialPropertyLib::VermaPruessModel::_phi_c

Documentation/ProjectFile/properties/property/VermaPruessModel/t_exponent.md

+\copydoc MaterialPropertyLib::VermaPruessModel::_n

Documentation/ProjectFile/properties/property/VermaPruessModel/t_initial_permeability.md

+\copydoc MaterialPropertyLib::VermaPruessModel::_k0

Documentation/ProjectFile/properties/property/VermaPruessModel/t_initial_porosity.md

+\copydoc MaterialPropertyLib::VermaPruessModel::_phi0

Tests/MaterialLib/TestMPLPermeabilityVermaPruessModel.cpp

+/**
+ * \copyright
+ * Copyright (c) 2012-2022, OpenGeoSys Community (http://www.opengeosys.org)
+ *            Distributed under a Modified BSD License.
+ *              See accompanying file LICENSE.txt or
+ *              http://www.opengeosys.org/project/license
+ *
+ * \file
+ *
+ * Created on November 2, 2016, 3:09 PM
+ */
+
+#include <gtest/gtest.h>
+
+#include <vector>
+
+#include "MaterialLib/MPL/Properties/CreateVermaPruessModel.h"
+#include "MaterialLib/MPL/Properties/VermaPruessModel.h"
+#include "MaterialLib/MPL/Utils/FormEigenTensor.h"
+#include "ParameterLib/ConstantParameter.h"
+#include "TestMPL.h"
+#include "Tests/TestTools.h"
+
+TEST(MaterialPropertyLib, PermeabilityVermaPruessModel)
+{
+    ParameterLib::ConstantParameter<double> const k0("permeability_ini",
+                                                     1.5e-14);
+    ParameterLib::ConstantParameter<double> const phi0("porosity_ini", 0.46);
+    ParameterLib::ConstantParameter<double> const phi_c("critical_porosity",
+                                                        0.3956);
+    ParameterLib::ConstantParameter<double> const n("exponent", 5);
+
+    auto const k_model = MPL::VermaPruessModel(k0, phi0, phi_c, n);
+
+    ParameterLib::SpatialPosition const pos;
+    double const t = std::numeric_limits<double>::quiet_NaN();
+    double const dt = std::numeric_limits<double>::quiet_NaN();
+
+    std::vector<double> const porosity{0.44, 0.38};
+    std::vector<double> const k_expected{2.3365591132239605e-15, 0.};
+
+    MPL::VariableArray vars;
+
+    for (std::size_t i = 0; i < porosity.size(); ++i)
+    {
+        vars[static_cast<int>(MaterialPropertyLib::Variable::porosity)] =
+            porosity[i];
+
+        auto const k = MPL::formEigenTensor<1>(k_model.value(vars, pos, t, dt));
+
+        // Compare the calculated value with the expected value.
+        if (k_expected[i] == 0.)
+        {
+            ASSERT_EQ(k_expected[i], k(0, 0));
+        }
+        else
+        {
+            ASSERT_LE(std::fabs(k_expected[i] - k(0, 0)) / k_expected[i],
+                      1e-10);
+        }
+    }
+}