(BUGFIX) calculate triangle area without using numpy in cython

Ideally this should all be done using numpy vector operations

Author: lachlangrose

LoopStructural/interpolators/cython/dsi_helper.pyx

@@ -70,7 +70,7 @@ def cg(double [:,:,:] EG, long long [:,:] neighbours, long long [:,:] elements,d
             # 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*np.linalg.norm(norm)
+            area = 0.5*sqrt(norm[0]*norm[0]+norm[1]*norm[1]+norm[2]*norm[2])#np.linalg.norm(norm)
             for itr_left in range(Na):
                 idc[ncons,itr_left] = idl[itr_left]
                 for i in range(3):
@@ -235,7 +235,7 @@ def fold_cg(double [:,:,:] EG, double [:,:] X, long long [:,:] neighbours, long
             norm[0] = v2[2]*v1[1] - v1[2]*v2[1]
             norm[1] = v1[2]*v2[0] - v1[0]*v2[2]
             norm[2] = v1[0]*v2[1] - v1[1]*v2[0]
-            area = 0.5*np.linalg.norm(norm)
+            area = 0.5*sqrt(norm[0]*norm[0]+norm[1]*norm[1]+norm[2]*norm[2])#np.linalg.norm(norm)
 
             i = 0
             for itr_left in range(Na):