Implement part of ParallelStateSpaceExplorer for #54

* Move rate calculation from sequential algorithm to the utils
* Add useful method just to getSuccessors() which returns only the
  Collection of states
* Implement the Callable class ParallelStateExplorer and add unit
* tests.  This class will not yet work becuase it relies on a single
  ExplorerUtilites which alters the Petri nets underlying state. Thus
  in parallel race coniditons will exist. #56 should fix this
* Have not finished/tested the parallel implementation