Remove HelicityAngles and RequiresHelicityAngles #116
Description
Since the helicity angles are only needed to calculate spin amplitudes in the helicity formalism(s), which are only calculated at the loading of data, we don't need to keep them in the DataPoint. We can instead have HelicityFormalism calculate fresh for each data point as its loaded, and this can be used by the HelicitySpinAmplitude's.