Reinitialize prior dict when reconstructed from json

When reconstructing the PriorDict object from a json file, some of the attributes were not reconstructed properly, possibly due to the fact that they were not encoded in the json file when calling .to_json(). This causes parallel_bilby_analysis to crash since it reconstructs the priors to be used from a json file generated earlier by parallel_bilby_generation.

The solution to this would be simply re-initializing the PriorDict object in .from_json().