[MeL] Add getBaseNodes() collecting nodes from elements.

The vector is filled with pointers to base nodes.

MeshLib/Elements/Utils.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
+ *
+ */
+
+#ifndef MESHLIB_ELEMENTS_UTILS_H_
+#define MESHLIB_ELEMENTS_UTILS_H_
+
+#include <algorithm>
+#include <vector>
+
+#include "BaseLib/makeVectorUnique.h"
+#include "MeshLib/Node.h"
+
+#include "Element.h"
+
+namespace MeshLib
+{
+/// Returns a vector of node pointers containing the base nodes of the elements
+/// input vector.
+inline std::vector<Node*> getBaseNodes(std::vector<Element*> const& elements)
+{
+    std::vector<Node*> base_nodes;
+    base_nodes.reserve(elements.size() * 2);  // Save some of the realloctions.
+
+    for (auto* const e : elements)
+    {
+        std::copy(e->getNodes(), e->getNodes() + e->getNumberOfBaseNodes(),
+                  std::back_inserter(base_nodes));
+    }
+
+    BaseLib::makeVectorUnique(base_nodes);
+
+    return base_nodes;
+}
+
+}  // namespace MeshLib
+
+#endif  // MESHLIB_ELEMENTS_UTILS_H_