Expose an option to run the procedure sequentially in explain.

[jonlachmann]

For debugging purposes it seemed beneficial to be able to use the non-parallel version of the code. It is also useful if many parallel explanations are run, to avoid over-subscription of the available cores (i.e. running more threads than cores, causing slowdowns).

[martinju]

Thanks! I agree this is useful for devel purposes (which is the reason for putting it in there in the first place). However, I am unsure whether it is wise to add another argument to explain(), as there are already quite many. I will leave this open for now, while thinking it through.

[martinju]

Closing for now as only useful for development (and there are already many arguments in explain())