#1 Extra Methods to configure how threads are created

[Rhys Newman]

Great to see use of the java.util.concurrent ExecutorService to create the necessary threads, however I need to be able to control thread creation in more detail. So I've opened up the ConcurrecyUtils class a bit to allow me to set the ExecutorService and/or ThreadFactory I want before I start calling the FFT functions.

The original way of creating threads is retained for people who don't need this functionality. I'm open to additional suggestions on the best way to do this, but the attached file seemed sensible.

[Rhys Newman]