Ingore invalid argument to arccos when finding stack in segmentmodel_from_lists

mrdna/readers/segmentmodel_from_lists.py

@@ -67,7 +67,8 @@ def find_stacks(centers, transforms):
     _z = np.array((0,0,1))
     for i in np.unique(idx1):
         js = idx2[ idx1 == i ]
-        angles = [np.arccos( transforms[j].T.dot( transforms[i].dot(_z) ).dot( _z ) ) for j in js]
+        with np.errstate(divide='ignore',invalid='ignore'):
+            angles = [np.arccos( transforms[j].T.dot( transforms[i].dot(_z) ).dot( _z ) ) for j in js]
         angles = np.array( angles )
         tmp = np.argmin(dists[i][js] + 1.0*angles)
         j = js[tmp]