fix: normalise vector for cg

LoopStructural/interpolators/cython/dsi_helper.pyx

@@ -25,6 +25,7 @@ def cg(double [:,:,:] EG, long long [:,:] neighbours, long long [:,:] elements,d
     cdef double [:,:] e1
     cdef double [:,:] e2
     cdef double area = 0
+    cdef double length
     cdef long long [:] idl  = np.zeros(4,dtype=np.int64)
     cdef long long [:] idr = np.zeros(4,dtype=np.int64)
     for e in range(ne):
@@ -67,10 +68,13 @@ def cg(double [:,:,:] EG, long long [:,:] neighbours, long long [:,:] elements,d
             norm[1] = v1[2]*v2[0] - v1[0]*v2[2]
             norm[2] = v1[0]*v2[1] - v1[1]*v2[0]
 
+            length = np.linalg.norm(norm)
             # we want to weight the cg by the area of the shared face
             # area of triangle is half area of parallelogram
             # https://math.stackexchange.com/questions/128991/how-to-calculate-the-area-of-a-3d-triangle
-            area = 0.5*sqrt(norm[0]*norm[0]+norm[1]*norm[1]+norm[2]*norm[2])#np.linalg.norm(norm)
+            area = 0.5*length#sqrt(norm[0]*norm[0]+norm[1]*norm[1]+norm[2]*norm[2])#np.linalg.norm(norm)
+            for i in range(3):
+                norm[i]/=length
             for itr_left in range(Na):
                 idc[ncons,itr_left] = idl[itr_left]
                 for i in range(3):