functor improvements

• Move helper classes for iterating over Pr::Velo::Tracks into Event/TrackEvent/Event/PrIterableTracks.h (closes Rec#72 (closed))
• Pick up updated SOAContainer (needed for accompanying changes in Rec)
• Add some helper functions to SIMDWrapper to allow the vector instruction set used when building the stack (stackInstructionSet()) and "now" (instructionSet()). These are useful for jitted functors to allow warning messages when scalar functors are mixed with a vectorised stack build, and do not interfere with the core SIMDWrapper classes.
• Add closestToBeamState() accessor to v2::Track

Goes with Rec!1625 (merged) and Moore!204 (merged).

cc: @sstahl @ahennequ