Support dumping and loading from JSON/YAML in gaudirun.py
As discussed in #196 (closed), having an 'options dump' format which one can run from is very useful. Currently only pickle dumps can be run from, but we would prefer to drop pickle usage entirely and so require supporting a different format.
Dumping to JSON or YAML would not be too hard to implement and it should be feasible to support running from such a dump.
I don't have too strong a preference on JSON vs. YAML. I prefer writing YAML but these dumps should never (?) be written by hand, so I'd err towards JSON where the tooling support is a bit larger and the format much simpler.