Add BtoKpipigamma model in AmpGen

Updated Gen/AmpGen to:

• include custom spin factors for B-> Kres gamma decays (this requires setting an additional parameter in the model .opt file, so would not affect any existing models/samples).
• add a final state density matrix for cases in which there are two polarisation states.

Also added two B->Kpipigamma decay models in Gen/LbAmpGen.

To-do: Compare generated models against those produced with equivalent AmpGen fitter (based on GooFit/AmpGen)