Implement delta-R based track-to-jet association as a decorator

Needed both for quick studies and potentially for HLT operations where rerunning jet finding for the sake of track association is undesirable overhead.

Ensures a 1:1 track-to-jet assignment by finding the nearest jet to each track. Not guaranteed to reproduce results of ghost association.

Also fixed a potential crash that occurred on PFlow jets due to constituents being absent. This was because constituent tracks were searched for even when the calculation of moments based on charged PFOs was switched off.