WIP: Added option to provide no events to an MCMC run, for prior sampling

This was a very minor change. In the CLI parsers I changed "+" (meaning one-or-more) to "*" (meaning any number) as the number of arguments to the "events" variable-length CLI argument. The rest of the code infrastructure already just looped over the list, and the base-case of an empty list was handled correctly.

This has been marked WIP because the more significant !5 (closed) was included in this, and I would like that to be resolved beore this is merged.