FFT executor service

Hello all!


Today @dietzc and I converted FFTConvolution(, FFT, FFTMethods) to use ExecutorServices.

FFT and FFTMethods got convenience methods to support a number of threads parameter,
FFTConvolution was converted completely but got a utility method to create an ExecutorService.

@StephanPreibisch @tpietzsch:
You may want to review. Main issue still is to maybe find a better way to determine number of threads (see FIXMEs in FFTMethods).


Greeting,
Jonathan Hale (Squareys)