Observed=False for df groupby: Retains functionality

Observed=False for df groupby: Retains functionality, observed=True will be default in future (depracation warning). Silences annoying warnings.

Change to https://gitlab.cern.ch/cms-analysis/general/HiggsDNA/-/blob/master/higgs_dna/tools/decorrelator.py?ref_type=heads#L93.

See also https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.groupby.html