Cost normalisation by non-zero mask area.

src/tirl/costs/msd.py

@@ -154,7 +154,7 @@ class CostMSD(Cost):
         if combined_mask is not None:
             costs = costs * combined_mask[nonzero]
         if self.metaparameters.get("normalise"):
-            return np.sum(costs) / stensors.shape[-1]
+            return np.sum(costs) / np.count_nonzero(nonzero)  # mod 13 Aug 2020
         else:
             return np.sum(costs)