Functional rates with the ability to select an entire place's tokens
or a particular token
Very basic functional weights working on arcs

* No error checking for floats etc yet
Modify FunctionalWeightParser to take an expression in the evaluate
method
Fix typo in "addAnnotaiton" method
Add test for functional rates to PetiNetTest

* Fix PetriNet up so that it only increments/decrements places
  after calculating incidence matricies
Comment up PetriNet
FunctionalRate Error messages appear when editing arcs

* Add unit tests