strip-only triplet-based seeding steps

Switched off by default for now (no difference is expected from this pull). 
Results presented here: https://indico.cern.ch/getFile.py/access?contribId=4&resId=0&materialId=slides&confId=261789
Short summary of code changes below.

** RecoTracker/TkSeedGenerator ** 
Algorithm implemented in RecoTracker/TkSeedGenerator/plugins/MultiHitGeneratorFromChi2.
MultiHitGenerator classes are a generalization of corresponding HitTripletGenerator one; they create OrderedMultiHits, i.e. SeedingHitSets with undefined number of hits (thus allowing both triplets and pairs from the same generator).

** RecoTracker/TkSeedingLayers **
Adding RecoTracker/TkSeedingLayers/interface/OrderedMultiHits.h and adding zero size case to SeedingHitSets

** RecoTracker/IterativeTracking **
Adding python files for triplet-based iter5 and iter6 (both in default and PostLS1 versions):
RecoTracker/IterativeTracking/python/PixelLessTripletStep_cff.py
RecoTracker/IterativeTracking/python/TobTecHybridStep_cff.py
They are included but commented out in iterativeTk_cff.py

** RecoPixelVertexing/PixelTriplets **
moving files from plugins to interface,src so that the library with the needed symbols can be used in other packages (i.e. RecoTracker/TkSeedGenerator)