Task parallelism

Experimental and off by default: enable it with --enable-patterns task (the default is reduction,doall). The detector is incomplete.

The task detector (new_task_detector.py in the explorer) works on the task graph that is built on top of the program execution graph. It identifies simple tasking and reports the results in the task list of .discopop/explorer/patterns.json; the entries are TaskParallelismInfo and ParallelRegionInfo objects. The remaining detection steps are not implemented yet. For example/example.cpp, --enable-patterns task reports no task pattern (the task list is empty).

The pattern * enables all detectors, including this one; the run can then take much longer than with the default.