Added default values for mistag (0.5) and b flavour decision (0).

@cprouve @bkhanji @vchobano I added some default values to the tagging class which I used in the flavour tagging ThOr functors and functor tests when it fails to find appropriate tagging information for the B. Where I used them in Rec:

https://gitlab.cern.ch/lhcb/Rec/-/blob/dtou_ft/Phys/FunctorCore/include/Functors/Composite.h#L265 https://gitlab.cern.ch/lhcb/Rec/-/blob/dtou_ft/Phys/FunctorCore/tests/src/TestFunctors.cpp#L1280

Rec!2854 (merged) depends on this.