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.

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1. FIX: use AmpForm's FormFactor #148
   ⚠️ Interface 🔨 Maintenance +2
   
2. Add p_l in energy-dependent width
3. Speed up notebook with parametrized function (skip additional xreplace() steps)