Better variable binding

[hfoffani]

This library provides two main features, the PDDL parser and a variable binding mechanism.

The current one is very naive and does not scale up.

Ideally, besides enhancing the algorithm it should provide the user a way to replace or overload this function with a custom one if desired.

[hfoffani]

Version 0.2.0 includes a better binding. It scales up a lot more than the previous version.

There are some optimizations left, some easy like trading memory space for speed, but others are more complicated and probably besides the scope of this tool. For instance, check the pre-condicontions as soon as each variable is grounded and if the operator cannot be applied cut the whole subspace for that variable-symbol pair.