ReDecay support for GenXicc and BcVegPy

Requires !358 (merged) This modifies the configuration to accomodate Special as a sample generation tool:

• The ReDecay instance uses SignalPlain as usual, but without the decay tool specified
• Signal decays rely on the implementation in the cut tool (which is just copied from Generation().Special
• Signal PIDs (required for SignalPlain) are assigned from the production tool set with Special (experimental)