make prior reading go through from_dict class

I noticed that there is still an eval statement in the PriorDict class. This is a relic of an old way of reading in priors.

I got rid of it by moving the logic in from_file into from_dict. This now means that creating the prior from either a text file or a json file goes through the same method.

I had to modify a test as it's now not possible to pass a string into the PriorDict in this way. Do people want to keep that possible?

Previously it was possible to make a prior from a dictionary that contained strings, e.g., waveform approximants, and it would just leave it as a string.