Formulate kinematics variables symbolically with SymPy and AmpForm

[shenvitor]

Formulate the conversion from four-momenta to Mandelstam variables and helicity angles symbolically. This is required to visualize the model over a phase space sample.

[redeboer]

issue to be solved in DOC: form amplitude models in symbolic expression for PWA101 channel #13

Probably better to only formulate the amplitude model in that PR for now. You can then address this issue through a separate PR where you visualize the model (for which you need a momentum-to-angle DataTransformer).

[redeboer]

For the implementation, have a look at this:
https://ampform.readthedocs.io/0.15.x/usage/kinematics.html#lorentz-vectors