Add policy checker?

I think a core and superb tool to add here is a policy checker/validator. I know you did one for PRP @haz but if I remember it was not efficient or generalizable? Maybe I am mixing things up...

In any case, a FOND validator is of course much more challenging than one for classical planning, but a much needed one when one does work in FOND. One of the things needed to do this is to standardize a policy output I guess?

anyway, before I forget I wanted to open this issue to discuss this desired feature in this repo. Seems to me it is *the* repo to host this tool.