introduce IReentrantSystematicsTool
This splits up the current ISystematicsTool into two interfaces, one part from which tools can inherit if they want to implement systematics in a thread-friendly manner, and one which contains the current interface and inherits from the former. See interface documentation for more details.
We discussed that sometime last summer, so this is now a draft of how that would look like in practice to give people a chance to say if they like it or not.
Edited by Nils Erik Krumnack