Adding support for non parameterized predicates and negative preconditions

[Momofil31]

What is this

This will add support for non parameterized predicates in pddl meaning that you can hardcode a value in a predicate, useful in preconditions and effects of actions for example.

How it works

Predicates are in the form of:
(<predicate_name> <argument_1> ... <argument_n>)

For example:
(in_room ?r)

Where ? is used to mark the argument as a parameter.
By removing the ?, the argument becomes an hardcoded value. For example:
(in_room kitchen)

Commit 97aacd8 introduce this feature by checking if the argument is present in the parameters list and adding the ? accordingly, when creating the predicate string.

Commit 4403ad9 simply adds support for negative preconditions in pddl.