Minor updates to plotting and base_config

MR description

This MR introduces the following two modifications:

• the plotting style is slightly modified to have a nicer looking uncertainty band in the ratio plots. The new implementation produces bands that have the following look:

  • for stat. only uncertainty: dnn_HHbbtt_Stat
  • for stat.+syst. uncertainty: dnn_HHbbtt_Stat+Syst

• the base config is initialised with the tau_id and bjet_id working points defaulted to an initialisation value and add_systematics is called before add_features

In both cases the changes have no impact on older installations of the code.

MR Validation

The MR has been extensively validated by running the following two scripts that run a full testing pipeline from PreprocessRDF to CreateDatacards: testSystematics2022.sh and testSystematics2023.sh