Added translator, improved generality and robustness

We made a couple of additions and improvements:

• We externalized the PDDL normalization step (translator) from Mimir into Loki. The normalization of function expressions, nondeterministic and probabilistic effects is still missing.
• For generality, we reworked the parser and translator to work with collections of problems. The domain can now be shared among problems and additional problems can be parsed and translated at any time.
• For robustness we added builders for domain and problem that do additional verification to ensure that planners can rely on indexing schemes where it is possible, i.e., objects, predicates, axioms, actions, function skeletons.