Add missing 'vol_diff' key to BinaryPairwiseMeasures.measures_dict

Otherwise, the following KeyError is raised:

Traceback (most recent call last):
  File "/Users/user/code/MetricsReloaded/MetricsReloaded/metrics/pairwise_measures.py", line 1161, in to_dict_meas
    if len(self.measures_dict[key]) == 2:
KeyError: 'vol_diff'

MetricsReloaded/metrics/pairwise_measures.py

@@ -272,6 +272,7 @@ def __init__(
             "hd_perc": (self.measured_hausdorff_distance_perc, "HDPerc"),
             "masd": (self.measured_masd, "MASD"),
             "nsd": (self.normalised_surface_distance, "NSD"),
+            "vol_diff": (self.vol_diff, "VolDiff"),
         }
 
         self.pred = pred