[MeL] Add polymorphic construct to Element class.

The existing clone function is a direct copy, but
setting different nodes is required.

MeshLib/Elements/Element.h

@@ -172,6 +172,14 @@ public:
      */
     virtual Element* clone() const = 0;
 
+    /**
+     * Constructs a new object polymorphically. This is similar to clone, but
+     * accepts new nodes and id.
+     * \pre The length of the \c nodes vector is equal to the derived element's
+     * total number of nodes.
+     */
+    virtual Element* clone(Node** nodes, std::size_t id) const = 0;
+
     /**
      * Computes the length / area / volumen of this element. This is automatically
      * done at initalisation time but can be repeated by calling this function at any time.

MeshLib/Elements/TemplateElement.h

@@ -69,6 +69,12 @@ public:
         return new TemplateElement(*this);
     }
 
+    /// \copydoc MeshLib::Element::clone(Node*[], std::size_t)
+    virtual Element* clone(Node** nodes, std::size_t id) const
+    {
+        return new TemplateElement(nodes, id);
+    }
+
     /// Get dimension of the mesh element.
     unsigned getDimension() const { return dimension; }