Update _rotations.py

Make sure that arccos is never given a value outside of its allowable range. The previous version would occasionally yield a nan for stiff rods.

elastica/_rotations.py

@@ -146,9 +146,12 @@ def _inv_rotate(director_collection):
             )
         )
 
-        # TODO HARDCODED bugfix has to be changed. Remove 1e-14 tolerance
-        theta = arccos(0.5 * trace - 0.5 - 1e-10)
+        # Clip the trace to between -3 and 3. It any deviation beyond this is numerical error
+        trace = min(trace,3.0)
+        trace = max(trace,-3.0)
+        theta = arccos(0.5 * trace - 0.5 )
 
+        # TODO HARDCODED bugfix has to be changed. Remove 1e-14 tolerance
         vector_collection[0, k] *= -0.5 * theta / sin(theta + 1e-14)
         vector_collection[1, k] *= -0.5 * theta / sin(theta + 1e-14)
         vector_collection[2, k] *= -0.5 * theta / sin(theta + 1e-14)