Possible optimizations

[francescofuggitti]

Some optimizations could be implemented to get plan4past work faster.

1. As the compiler works on the syntactic structure of the PPLTL formula, a preprocessing to simplify the input formula could improve the performances
2. Simplify the model of actions and axioms by checking the monotonicity of formulas. For instance, when O(a) becomes true, then it can not be falsified in the future. This implies that the conditional effect \neg ppnf(O(a)) -> \neg "Y(O(a)) is redundant and not needed.