FEAT: describe initial/final state with State class

[redeboer]

Closes #108

• Implemented a State data structure, which derives from Particle and has an index. This allows distinguishing particles in the final state if they are equal.
• It also made it possible to generalize the dynamics builders. They were defined in the notebooks so far, but have been generalized and moved to a ampform_dpd.dynamics.builder module. See also FEAT: define RelativisticBreitWigner and FormFactor #113.
• A helper function permutate_equal_final_states() was added for QRules state transitions with equal particles in the final state.