add option config shifting the maxZ to consider for SplitSim photon conversion productions at TT stations locations

Add an additional python script to enable productions to optimise for converted photons , allowing also DownDown electrons to be considered as part of the production. The default SplitSim set the MaxZ at 500 mm, making the production useless to analysis doing the di-electrons from down-down categories. My plan was to submit a large request for Lb-> L gamma, Bs -> Phi gamma, B0 -> K*0 gamma using SplitSim allowing gamma->ee conversion anywhere before TT to have ee reconstructible as long or down tracks. @adavis (thanks for the suggestions on how to best do this!)

People which might be interested in having this functionality in production i am aware of: @mschune , @cprouve , @lcapriot (for exotics), @chefdevi

I locally have run a test production for 2016 data taking (100 events) with default settings for Bd_Kstgamma=DecProdCut.dec:# EventType: 11102201

And for MaxZ = 500 only 6/100 pass the filter, while for MaxZ=2500 i get 19/100, which sounds reasonable.

Is there any additional checks to be done to ensure this could be picked up in a production request ?