Use cases for `TaskManager` and `TaskRunManager`?

[drbenmorgan]

In preparing some material on the use of PTL for the Geant4 project and the LHCb experiment, I'm somewhat confused by the PTL::TaskManager and PTL::TaskRunManager classes are so wanted to clarify a few questions.

1. Most simply, what are the use cases for these classes vs direct use of PTL::TaskGroup and/or PTL::ThreadPool?
2. What's the case for having the default (static, global) threadpool instance used in PTL::TaskGroup managed by PTL::TaskRunManager rather than as a local static of a free function?
3. Are any of the member functions of PTL::TaskManager other than the async ones relevant now, or rather, why have trivial wrappers/reproducers of functions directly available in the interface of PTL::TaskGroup like wrap and exec?

I realise I'm probably missing something here, but am wondering if some simplification/reduction is possible. This is also relevant for experiments using TBB and making PTL's ability to use this as clear and straightforward to integrate as possible.