Improve plotting/plot_utils.py loader() function

[lucalavezzo]

Needs to be cleaned up a bit, originally designed to work for ggF offline. In particular, at the moment I can think of:

• [x] delete the removing of low ht bins code, this is a really old leftover
• [x] come up with a better way to define groups of samples that should be combined, e.g. the QCD_HT bins are nicely loaded separately AND combined into one QCD_HT, but I think we can improve how we do this, at the moment it's loose string matching
• [x] come up with a better way to assign 'nicknames' to samples. I don't want to type out "WJetsToLNu_HT-800To1200_TuneCP5_13TeV-madgraphMLM-pythia8+RunIISummer20UL18MiniAODv2-106X_upgrade2018_realistic_v16_L1v1-v1+MINIAODSIM" every time, so at the moment we were assigning short nicknames e.g. "WJetsToLNu", but this can also be improved / maybe a new standalone function

[lucalavezzo]

Above are addressed in #279