removing weight initialisation in prepare step

the weights are already initialised in the prepare step which is not optimal, the idea is to maintain the full file structure after that step and just filtering the different jets

so we need to change https://gitlab.cern.ch/atlas-flavor-tagging-tools/algorithms/umami/-/blob/master/umami/preprocessing_tools/Preparation.py#L147

and in addition we need to reomve the pandas data frames since they are a big performance killer https://gitlab.cern.ch/atlas-flavor-tagging-tools/algorithms/umami/-/blob/master/umami/preprocessing_tools/Preparation.py#L145

tagging here @frenner