Add robustness to PMCMC steps

[perrydv]

PMCMC could be updated to:

• check on valid priors before calling the particle filter likelihood approximation, and
• check for invalid results from the particle filter and reject the proposed parameters.

[paciorek]

The first item is in @danielturek PR from December.

@perrydv regarding the second time, were you thinking of checking for NA/NaN/non-finite likelihood estimates?

[perrydv]

@paciorek I took a look at the second point and I think no change is necessary. The decide function that ultimately returns the logical for the accept/reject outcome checks for is.na (which catches both NA and NaN), so that should cover the issue already.