Steer Run3 muon low energy and flat spillover background parametrisation from Boole properties

A new convention for the suffix of muon low energy and flat spillover background parametrisation files has been introduced, indicating the Run it refers to and the version: Run#-v#.#. For Run3 we have muonflat-G4-Run3-v2.0.root and muon low-G4-Run3-v2.0.root lhcb-datapkg/ParamFiles!118 (diffs) The purpose of this new convention is to steer the input files from Boole() properties.
To this extent some changes in Boole Digi/Boole/python/Boole/Configuration.py are proposed to allow a configurable setting of bg_suffix.

Further enhancements are discussed in this issue #31 (closed)

/cc @gcorti @edelucia