run treadmill on-demand and shutdown when no ugen uses it
This PR will switch to running the treadmill thread only when a ugen is actually using it.
All Ugens now have a destructor that will decrement the use counter for the treadmill and, if needed stops it.
In order to check whether this works, I used htop
(which may be installable on macOS via brew install htop
). This allows filtering processes by name and inspecting its thread (they show in a different color). Then I used a simple test to check if destructing e.g. OmegaButton leads to stopping of the thread.
Please test on real hardware...
closes #25