Threshold before computing sqrt for kappa

For noisy data, the numerical errors can lead to tiny positive
values in the Hessian, then giving NaN in the sqrt. So, we threshold.

SIRF_data_preparation/create_initial_images.py

@@ -82,9 +82,8 @@ def compute_kappa_image(obj_fun, initial_image):
 
     WARNING: Assumes the objective function has been set-up already
     """
-    # This needs SIRF 3.7. If you don't have that yet, you should probably upgrade anyway!
-    Hessian_row_sum = obj_fun.multiply_with_Hessian(initial_image, initial_image.allocate(1))
-    return (-1 * Hessian_row_sum).power(.5)
+    minus_Hessian_row_sum = -1 * obj_fun.multiply_with_Hessian(initial_image, initial_image.allocate(1))
+    return minus_Hessian_row_sum.maximum(0).power(.5)
 
 
 def main(argv=None):