FFTConvolution doesn't shutdown the ExecutorService that it creates

[acardona]

At https://github.com/imglib/imglib2-algorithm-gpl/ we see how the constructor invokes FFTConvolution.setExecutorService( service ) which works fine as long as the service is not null. When null, setExecutorService( service ) will create a new ExecutorService that will never be shutdown.

Upon repeated instantiation of FFTConvolution, the non-shutdown ExecutorService accummulates native memory, which can be seen as an accumulation of suspended threads, and eventually the JVM runs out of it.

The solution: a flag should be set when FFTConvolution instantiates a new ExecutorService, so that, upon completion of the main method convolve doing all the work, the service is shutdown().

Then, upon re-invokation of convolve, if the service is shutdown it should be reinstantiated--so this check should be done always at the beginning of convolve by asking ExecutorService.isShutdown(). Or it should thrown an error.