Reading an AmplitudeModel without form factors results in an AmplitudeModel with form factors

[redeboer]

Bug description

When importing a recipe file without form factors, the resulting AmplitudeModel contains form factors again.

How to reproduce?

import expertsystem as es

result = es.generate_transitions(
    initial_state=("J/psi(1S)", [-1, +1]),
    final_state=["gamma", "pi0", "pi0"],
    allowed_intermediate_particles=["f(0)"],
    allowed_interaction_types="strong and EM",
)
model = es.generate_amplitudes(result)
for name in result.get_intermediate_particles().names:
    model.dynamics.set_breit_wigner(name)

# Set form factors to None
for dyn in model.dynamics.values():
    dyn.form_factor = None

es.io.write(model, "model.yml")
imported_model = es.io.load_amplitude_model("model.yml")

# Check if form factors are still None
for dyn in model.dynamics.values():
    assert dyn.form_factor is None

Expected behavior

In the snippet above

for dyn in model.dynamics.values():
    assert dyn.form_factor == None

should work.