Add mbb regression into HHbbgg processor

MR adds DNN based dijet mass (mbb) regression into HHbbgg processor. This was previously implemented as a post-processor to HiggsDNA parquets, but adding it within the HiggsDNA processor allows for the regression to be applied to the entire dijet collection before the 70-190 GeV dijet mass cut.

Adds the regressed mass as a new dijet variable '{ANType}_dijet_mass_DNNreg', and also includes a new category 'nonResReg' for dijets cut on the regressed mass instead of reconstructed mass, while also keeping the Res and nonRes collections for comparison.

Tested on a GluGluHH signal sample and GGjets background, comparison between regressed dijet mass and reconstructed dijet mass for each collection here: plots