Add the option to choose how many threads

* If we choose 1 then the StateSpaceLoader uses a sequential explorer
  any other value is done via the MassiveParallelStateSpaceExplorer
* No error checking yet if a non integer value is added
Fix typo
Share the code to choose the number of threads and the go button #73

* Error check the number of threads
Add javadoc to gobutton
Merge pull request #74 from sarahtattersall/feature_73

Feature 73