Allow setting dynamics by IsobarNode instead of complete decay chain

[redeboer]

Currently, one sets dynamics for the complete chain, e.g.:

model_builder = DalitzPlotDecompositionBuilder(DECAY, min_ls=True)
for chain in model_builder.decay.chains:
    model_builder.dynamics_choices.register_builder(
        chain, formulate_breit_wigner_with_ff
    )
model = model_builder.formulate(reference_subsystem=1)

In AmpForm, one sets dynamics for each isobar node separately, e.g. a form factor only for the production node and a relativistic Breit-Wigner with form factor for the decay node.