FFTMethods now uses ExecutorService

Added methods which use a given ExecutorService.
Methods which have number of threads given create a ExecutorService,
default number of threads is set to:
	Runtime.getRuntime().availableProccessors()
FFT now uses ExecutorServices

Added methods which use a given ExecutorService. Methods wich have
a given number of threads now create an ExecutorService. Default
number of threads is set to:
	Runtime.getRuntime().availableProcessors()