Gauss using ExecutorService

Hi,

some weeks ago @tpietzsch and me were looking into some of the multi-threaded algorithms and added the possibility to use a ExecuterService provided from outside. This has several benefits, especially if the program using the algorithm maintains his own thread management one can control how many threads are used by the algorithm. 

@tpietzsch you think we can merge?

@StephanPreibisch do you think we can adopt these ideas in the FFT classes? This would be very nice. Currently there are many threads created over and over again. Imagine you run fft on lets say 10000 images using the same jvm. at some point the jvm simply gives up garbage collection / maintaining all those created threads and crashed.