Flexible ParallelComputer

The ParallelComputer with unlimited Thread pool size is not enugh.

Thus I added methods with parameters where the user can change the Thread pool as he likes.

Opened discussion #518 and got the consensus to update ParallelComputer.

methods(ExecutorService)
classes(ExecutorService)
classesAndMethodsUnbounded()
classesAndMethodsBounded(int nThreads)
classesAndMethods(ThreadPoolExecutor)
classesAndMethods(ThreadPoolExecutor pool, int minConcurrentMethods)
classesAndMethods(ExecutorService poolClasses, ExecutorService poolMethods)