cos_elem -> cos_nod

cos_elem is used for the scaling of edge length. It lead to a random behaviour: the cos_elem of the first neighbouring element is taken. If the mesh is resorted, the result would be slightly different. Now, we employ cos_nod, which is a) cheaper to compute, and b) reproducible for any mesh sorting. For the edge with nodes nod1, nod2, we now consider .5*(cos_nod(nod1)+cos_nod(nod2)).