Fix issue with cached labels in python label_dict wrapper (#1541)
Fix issue with cached labels in python label_dict wrapper (#1541)

The `label_dict` Python wrapper caches label strings and their descriptions in order to
provide a convenient Python iterator interface and `[]` access. These were not being
properly updated or set when the label_dict was constructed from a C++ label_dict.
halfflat/arbor,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/arbor,halfflat/nestmc-proto,halfflat/nestmc-proto,halfflat/nestmc-proto,eth-cscs/nestmc-proto,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/arbor