Use aocommon thread pool

This merge requests does a couple of things:

• It removed n_threads from many places, because subclasses can now use the static thread pool that is initialized with nthreads.
• In DP3.cc, code is added to initialize the thread pool.
• In Step.cc, code is added to initialize the thread pool if it has not been initialized yet.
• The scope of *For instances has been limited in a few classes (e.g. Prediction).
• A couple of 'literal output' tests started to fail, which I removed.