Fix validation checks and Dalitz plot in serialization example

[redeboer]

#132 implements a preview of deserializing a JSON amplitude model for . The amplitude builder works, but the numerical checks fail for now. Performing an intermediate check on the dynamics may help to find the problem, see RUB-EP1/amplitude-serialization#44.

• FIX: use AmpForm's FormFactor #148
• Add p_l in energy-dependent width
• Speed up notebook with parametrized function (skip additional xreplace() steps)