Added test.

Tests/NumLib/TestFunctionInterpolation.cpp

+/**
+ * \copyright
+ * Copyright (c) 2012-2015, OpenGeoSys Community (http://www.opengeosys.org)
+ *            Distributed under a Modified BSD License.
+ *              See accompanying file LICENSE.txt or
+ *              http://www.opengeosys.org/LICENSE.txt
+ */
+
+#include <gtest/gtest.h>
+
+#include <array>
+
+#include "NumLib/Function/Interpolation.h"
+
+TEST(NumLibFunctionInterpolationTest, TwoVariablesTwoNodes)
+{
+    double v1 = 0.0, v2 = 0.0;
+    std::array<double*, 2> interpolated_values = { &v1, &v2 };
+
+    const std::array<double, 4> nodal_values = {
+        0.0, 1.0, // v1
+        -1.0, 1.0  // v2
+    };
+
+    const std::array<double, 2> shape_matrix = { 0.25, 0.75 };
+
+    NumLib::shapeFunctionInterpolate(nodal_values, shape_matrix,
+                                     interpolated_values);
+
+    ASSERT_EQ(0.75, v1);
+    ASSERT_EQ(0.5,  v2);
+}
+