no failure on non existent variables

[emil keyder]

Hi all, 

The FD translator currently doesn't warn or fail when a predicate in the precondition or effect of an action uses a variable that doesn't exist in the action parameters nor within any enclosing universal/existential quantifier. I've been bitten by this a couple of times and was thinking of adding a check for it, probably by passing the "parameters" object here:

https://github.com/aibasel/downward/blob/main/src/translate/pddl_parser/parsing_functions.py#L249

into the parse_condition and parse_effect methods, if necessary adding the additional variables when recursing down past a universal/existential quantifier, and raising an exception if there's a variable in a condition that isn't declared in the current scope. 

Is there any reason not to do this? If I made a PR for this would it be accepted? Any gotchas that I should be aware of or suggestions for a different approach?

Thanks, 

Emil