Fix issue 468 (Levenberg-Marquardt damping term)

The damping term doesn't blow up so rapidly when the refinement is fully
converged (i.e. objective stops changing)

scitbx/lstbx/normal_eqns_solving.py

@@ -256,7 +256,7 @@ def do(self):
       objective_new = self.non_linear_ls.objective()
       actual_decrease = objective - objective_new
       rho = actual_decrease/expected_decrease
-      if rho > 0:
+      if rho >= 0:
         if self.has_gradient_converged_to_zero(): break
         self.mu *= max(1/3, 1 - (2*rho - 1)**3)
         nu = 2
@@ -325,7 +325,7 @@ def do(self):
       rho = actual_decrease/expected_decrease
       if self.objective_decrease_threshold is not None:
         if actual_decrease/objective < self.objective_decrease_threshold: break
-      if rho > 0:
+      if rho >= 0:
         if self.has_gradient_converged_to_zero(): break
         self.mu *= max(1/3, 1 - (2*rho - 1)**3)
         nu = 2

scitbx/lstbx/tests/tst_normal_equations.py

@@ -179,6 +179,14 @@ def exercise_levenberg_marquardt(non_linear_ls, plot=False):
     print("\[Mu]=%s;" % iterations.mu_history.mathematica_form(), file=f)
     print("ListLogPlot[{g,\[Mu]},Joined->True]", file=f)
     f.close()
+  non_linear_ls.restart()
+  iterations = normal_eqns_solving.levenberg_marquardt_iterations(
+    non_linear_ls,
+    track_all=True,
+    gradient_threshold=0,
+    step_threshold=0,
+    n_max_iterations=200)
+  assert iterations.n_iterations == 200
 
 def run():
   import sys