Add function to unbiase PVs + bestPV function

Add a free-standing function to Particle class that takes the PrimaryVertexContainer and a Particle and gives back the unbiased Vertices as PrimaryVerticesas well as a free-standing function that returns the best PV for a particle.

Thanks to @graven for all the help.

Both new functions are used in Rec!3082 (merged) Goes together with Moore!1750 (merged)