Add missing vol_diff key to BinaryPairwiseMeasures.measures_dict

[valosekj]

This is a minor PR adding the 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'

---

Note: I also originally opened the same PR on Project-MONAI/MetricsReloaded: Project-MONAI#44. But it has not been merged yet.

[naga-karthik]

Ah I remember looking at this but realized that it was under the actual MetricsReloaded repo! Disappointed that it's not active :/

Thanks for this change anyway though!