Add Diphoton mass resolution to base processor

Addresses #41 (closed) Includes sigma_m_over_m in the output of HiggsDNA for base processor.

Formula: \frac{\sigma_m}{m} = \frac{1}{2} \sqrt{\left( \frac{\sigma_{E{,}1}}{E_1} \right)^2 + \left( \frac{\sigma_{E{,}2}}{E_2} \right)^2} Implemented as diphotons["sigma_m_over_m"] = 0.5 * numpy.sqrt((diphotons["pho_lead"].energyErr / (diphotons["pho_lead"].pt * numpy.cosh(diphotons["pho_lead"].eta)))**2 + (diphotons["pho_sublead"].energyErr / (diphotons["pho_sublead"].pt * numpy.cosh(diphotons["pho_sublead"].eta)))**2) as energy is not a field of the photons.

Validation plot attached. Sample used: store/mc/Run3Summer22EENanoAODv11/VBFHtoGG_M-125_TuneCP5_13p6TeV_amcatnlo-pythia8/NANOAODSIM/126X_mcRun3_2022_realistic_postEE_v1-v1/2820000/bf0f0dee-f7c3-4b59-ad3b-4909b3c2f004.root Plot looks reasonable. Seems to be roughly in agreement with 6.5 of https://www.research-collection.ethz.ch/handle/20.500.11850/579650.
mass_resolution_vbf_example.pdf