Name change for rho variable between Run2 and Run3 NanoAODs

[fiemmi]

The README currently recommends to do

sed -i 's/"fixedGridRhoFastjetAll"/"Rho_fixedGridRhoFastjetAll"/g' bamboo/bamboo/analysisutils.py

in order to run on Run3 NanoAOD, where the ρ variable is called Rho_fixedGridRhoFastjetAll. This clearly makes the code crash when attempting to read Run2 samples, which still have the old naming.
This needs either to be treated in the code or harmonized by future NanoAOD versions (maybe with coherent naming?)