Add missing `vol_diff` key to BinaryPairwiseMeasures.measures_dict

[valosekj]

This PR adds 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'

@p-j-smith, @csudre, could you please review this PR?