Construct and return the lowest-order discrete gradient matrix G using some edge and vertex information.
Construct and return the lowest-order discrete gradient matrix G using some edge and vertex information. We assume that edge_vertex lists the edge vertices consecutively, and that the orientation of all edges is consistent.If edge_orientation = 1, the edges are already oriented.
If edge_orientation = 2, the orientation of edge i depends only on the sign of edge_vertex[2*i+1] - edge_vertex[2*i].