introduce SystematicsCache helper class template

This should make it simpler for CP developers to implement a (close to) optimal version of an ISystematicsTool or IReentrantSystematicsTool. We do have a very specific recommendation on how that should be done, but it is not exactly fool-proof, particularly if you are trying to do it in a thread-safe manner.

This is based on a discussion I had with @christos a while back, who had suggested that it was actually not that straightforward and a helper class would be helpful.

I ended up modifying one of the muon tools because it was the one that most closely followed our actual recommendation, making the update to the new helper class a straightforward replacement. Not sure if the muon people have any reason to object. @yoyamagu?

cc @lheinric @alister