Handle priors as part of the result object and several other minor fixes

1. The prior is now saved as a dictionary where the values are strings which eval to a tupak prior object. Externally, this is just a convientient way see what prior was used as part of the result. Internally, the prior can be used in post-processing directly.
2. As part of the above change, the PriorSet is modified to handle input dictionaries where the value is a string representation of the prior.
3. Methods for reading and writing PriorSets are renamed for consistentency
4. The plot_corner method was updated so that the prior doesn't have to be passed in explicitly, but can use the stored prior.
5. Since it is no longer required, the .prior text file is no longer saved by default in run_sampler. However, the method remains and can optionally by called should the user which to make use of it.

Additional changes 6) Default outdir and labels are added to the result objects.