Adding support for functions and numeric fluents

[Flamak]

PDDL functions are similar to predicates (which can be seen as functions with the domain {0,1}. PDDL functions with continuous domain are often called numeric fluents. For the purpose of this task, we are only interested in supporting functions as numeric fluents (e.g. object fluents are not in the scope of this task).

For numeric fluents, we would like to support

• numeric expressions (prefix math)
• numeric effects (increase & decrease & assign)
• conditions (<, >, =)

Common examples for using continuous numeric fluents are fuel, energy, tension ... , for discrete fluents then capacity (elevators), discrete resources (machine that does at most 3 tasks in parallel), etc.

PDDL Reference https://planning.wiki/ref/pddl21/domain

When implanted, please create a PR with the proposed extension, so we can test it on its branch.