When using the tbb::task API one doesn't have control over tbb's threadpool. (or at least I didn't find a way...)
The tbb::task api is deprecated in tbb 2020 and removed in OneAPI so moving away from the current implementation would have been required in the future anyway.
This MR let's the
AlgFlowManager create its own
task_arena. The maximum number of threads is still controlled by the online application via TBB's