fix extraction when fixation are named with custom strings (as in feito)

models/bonemodel2.py

@@ -189,7 +189,8 @@ def solve(p={}):
 
     print('extracting results...')
     print(f'\tinternal energy = {solver.int_energy:.3f} N.mm')
-    for grpname in ['axis_pt1', 'axis_pt2', 'contact_pts']:
+    for fix in p['fixations']:
+        grpname = fix['name']
         v = np.array(post.probe('force_vector', grpname))
         v = np.reshape(v, (v.size//3, 3))
         v = np.sum(v, axis=0)