[Unsat] Added classes for capillary pressure model

MaterialLib/PorousMedium/UnsaturatedProperty/CapillaryPressure/CapillaryPressureSaturation.h

+/**
+ * \copyright
+ * Copyright (c) 2012-2016, 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:   CapillaryPressureSaturation.h
+ *
+ */
+
+#ifndef OGS_CAPILLARY_PRESSURE_SATURATION_H
+#define OGS_CAPILLARY_PRESSURE_SATURATION_H
+
+#include <string>
+
+namespace MaterialLib
+{
+namespace PorousMedium
+{
+/// Base class of capillary pressure models
+class CapillaryPressureSaturation
+{
+public:
+    virtual ~CapillaryPressureSaturation() = default;
+
+    /// Get model name.
+    virtual std::string getName() const = 0;
+
+    /// Get capillary pressure.
+    virtual double getCapillaryPressure(const double saturation) const = 0;
+
+    /// Get capillary pressure.
+    virtual double getSturation(const double capillary_ressure) const = 0;
+
+    /// Get the derivative of the capillary pressure with respect to saturation
+    virtual double getdPcdS(const double saturation) const = 0;
+};
+
+}  // end namespace
+}  // end namespace
+
+#endif /* OGS_CAPILLARY_PRESSURE_SATURATION_H */