Update PFlowCellEOverPTool to read bin boundaries from json

This MR updates the PFlowCellEOverPTool such that bin boundaries are also read in from a json file. This will allow us to flexibly choose a set of bins when deriving the particle flow reference parameters because the code doing the derivation will now write the bin boundaries to a json file. This will avoid bugs which were recently uncovered whereby bin boundary choices did not match in eflowRec and the code used to derive the reference bin parameters,

This tool is not used in any production workflow currently. It will be used for the first time once we have the new set of HL-LHC reference parameters derived. Currently it loads the data from a private test area - this will be updated to a cvmfs area for the production reference data.