Topic add vtx vars

As discussed with Pasquale, add ability to get back vtx variables for all vertices. Number of vertices is configurable via:
nVtxSaveInfo =cms.untracked.uint32(100) in the FlashggDiPhotonProducer python config file.
If a very large number is chosen, the size will the number of vertices. This size is accessible by the diphotoncandidate method:
unsigned int getnVtxInfoSize() 
Vtx variables are accessible with methods like:
float getLogSumPt2(unsigned int iVtx)
Small note: this can also be achieved with a std::map<float,int> (which gets sorted automatically upon insert), without need for an explicit comparison operator.