ability to drop user into a debugging session upon unexpected behavior

[paciorek]

In various cases, we print warnings to users (e.g., related to model initialization and unexpected logProb values (e.g., see our current checkLogProb) during MCMC sampling).

We have ways to put a user into an R context in which they can check the state of the compiled objects, but these are not easily/systematically available to the user.

For nCompiler, I think we may want something like the following -- provide a nimbleOption that if turned on (off by default) will cause certain situations to drop the user into an R context. I think this will help with cases where a user doesn't understand why they have NAs, NaNs, Infs, etc. in various situations (or even what variable has contains such values).

Just flagging this for future consideration.