Adding new class for the calculation of WignerD function. It calculates
the helicity angle in a different way. It produces the same result as
the previous version, but it is a bit more compact. And I'm sure that
the masses are assigned correct; The factor (2J+1) is now included in
the Amplitudes, therefore it is included in the integral; the class
Amplitude can now calculate its normalization (tested!); DPKinematics
also provides the spin of the particles