Move the new functors' iteration proxies for the POD track classes somewhere more visible

The following discussion from !1601 (merged) should be addressed:

• @sstahl started a discussion: (+2 comments)

  Can we put the definition of the proxies closer to the implementation of Pr::Velo::Tracks so that other pieces of code can use them more easily?

Currently these are hidden in Predicate.h, but they could be useful for other algorithms that use the POD track classes, such as LHCb::Pr::Velo::Tracks.