replace the rather vaguely named 'hasVertex' with the better description
canBeExtrapolatedDownstream -- it distinguishes objects which are
parameterized as tracks, which has 5 parameters (eg. q/p,tx,ty,x,y as a
function of z) or 4, in case there is no momentum estimate,  versus
objects which have an endVertex, and thus have are parameterized by 6
parameter, i.e.  (x,y,z) and (px,py,pz) and which should not be extrapolated
beyond their vertex position (but which may be extrapolated upstream.
In addition, these objects have a measured invariant mass whereas
track-like objects typically have an assigned mass hypothesis.