Formulate angles for all subsystems if decay does not have all subsystems

[redeboer]

Since #127, amplitudes are only formulated for the subsystems that exist in a decay. This is okay for the amplitudes, but you may still want to have access to the angular distributions of the non-existent subsystsems. In AmpForm, this is done through HelicityAdapter.permutate_registered_topologies().

The solution probably lies here:

ampform-dpd/src/ampform_dpd/__init__.py

Lines 98 to 102 in e43f66c

	for sub_system in _get_subsystem_ids(self.decay):
	chain_model = self.formulate_subsystem_amplitude(*args, sub_system) # type:ignore[arg-type]
	amplitude_definitions.update(chain_model.amplitudes)
	angle_definitions.update(chain_model.variables)
	parameter_defaults.update(chain_model.parameter_defaults)

We would need a switch that allows calling angle_definitions.update(chain_model.variables) for sub_system in (1, 2, 3).