photon 4-momentum correction validation

Some additions : 
- Photon Correction Validator
- fixing ptasym in the LegacyVertexSelector
- now storing the vertex index in the diphoton object, to ease a fair bit checking if the selected vertex is the primary vertex or not.

edit: 
- adding some cleanup done while memory leak hunting, some including changes concerning LegacyVertexSelector
- missing dRexclude so ptasym was always 0
- Accidental removal of some initalisation while removing couts (https://github.com/cms-analysis/flashgg/commit/9b0d997857049f5fc401c60cd2fa6362c0096609#diff-b1625d765cf6583fd4358a76bf13f0e2L319), ending up in zconv always being 0, changes are back in