Fix Cjr sign in 2D

This was buggy since used gmsh meshes were incorrectly oriented ...
Should check cells volume

src/mesh/MeshData.hpp

@@ -106,7 +106,7 @@ class MeshData
               int Rp1         = (R + 1) % cell_nodes.size();
               int Rm1         = (R + cell_nodes.size() - 1) % cell_nodes.size();
               Rd half_xrp_xrm = 0.5 * (xr[cell_nodes[Rp1]] - xr[cell_nodes[Rm1]]);
-              Cjr(j, R)       = Rd{-half_xrp_xrm[1], half_xrp_xrm[0]};
+              Cjr(j, R)       = Rd{half_xrp_xrm[1], -half_xrp_xrm[0]};
             }
           });
         m_Cjr = Cjr;