Use a global thread pool for all threads (except main and UI)

[jbendtsen]

There a a few classes that seem to create a whole bunch of threads on their own, in a totally unorganised fashion. ForkJoinSkeletonize2 in particular creates threads recursively, such that they never cleaned up properly. This leads to an exhaustion of system resources and an OutOfMemoryError during batch analysis - likely thread count before memory, but still.

[jbendtsen]

List of classes that likely create their own threads:

• AngioTool/SwingWorker.java (unused, deleted)
• features/ForkEigenValuesAtPoint2D.java
• GUI/MyImageView.java (only uses Thread.sleep(), no actual threading)
• RecursiveGaussianFilter/Parallel.java (unused, deleted)
• RecursiveGaussianFilter/RecursiveGaussianFilter.java (unused, deleted)
• Skeleton/ForkJoinSkeletonize2.java
• Utils/ForkShapeRoiSplines.java (only gets called once from a loop with 0 iterations, ie. unused)
• vesselThickness/EDT_S1D.java
• vesselThickness/ForkStep2.java