Override NodeElem::volume() to return 1

This does not change the existing behavior, since the base class
Elem::volume() implementation already returned 1 for NodeElems, but it
does document the behavior while also avoiding any overhead that might
have come from calling Elem::volume().

Author: jwpeterson

include/geom/node_elem.h

@@ -271,6 +271,14 @@ class NodeElem : public Elem
    */
   virtual Order supported_nodal_order() const override;
 
+  /**
+   * Elem::volume() returns the N-dimensional measure of an
+   * N-dimensional element. For N=2 "volume" is area; for N=1 "volume"
+   * is length, and for N=0 "volume" is cardinality. A set with 1
+   * point has 0-"volume" of 1.
+   */
+  virtual Real volume () const override { return Real(1); }
+
   virtual void connectivity(const unsigned int sc,
                             const IOPackage iop,
                             std::vector<dof_id_type> & conn) const override;