photon preselection v2

This pull request is work in progress as there is some incomplete features : 
- this pull request is intended as a follow up of the discussion in https://github.com/cms-analysis/flashgg/pull/10
- the previous implementation of the preselection is deleted
- now the preselection is done at the diphoton candidate level and applied on the configuration file
- the getChargedPFIso02 and related functions still need synchronisation with https://github.com/cms-analysis/flashgg/pull/19
- the charged PF iso cuts are not applied for the moment