Bugfix: cost Jacobian with masked images.

src/tirl/costs/mi.py

@@ -301,7 +301,7 @@ class CostMI(Cost):
 
         # Masked point should contribute no gradient force for the registration
         if combined_mask is not None:
-            jac[combined_mask.ravel(), ...] = 0
+            jac[np.isclose(combined_mask, 0, atol=1e-3).ravel(), ...] = 0
 
         # return -jac
         return jac