feat!: implement spin alignment with DOI:10.1155/2020/6674595

[redeboer]

Implemented generic, multi-body spin alignment in case of multiple decay topologies, following 10.1155/2020/6674595.

Closes #6, closes #10, closes #8

See problem description in TR-015 and usage example here. For an example with generated data, see TR-013.

To-do list

• Generate Wigner-D chain in Marangotto Eq.(45)
  feat!: implement spin alignment with DOI:10.1155/2020/6674595 #245
• Sum over product of all combinations in Eq.(45)
• Skip m = 0 in case of massless particles
• Rewrite sum combinations with sympy.Sum and a (unique!) Dummy index.
• Compute Wigner rotation angles α, β, γ
  Calculate alignment angles of different subsystems #9
• Check if same procedure applies for formalism="canonical-helicity"
• Test if distributions are the same if a different reference Topology is chosen.
  docs: add TR-013 with spin alignment data test compwa.github.io#111
• Implement opposite helicity determination.
• Collect chain of helicity rotations.
• Formulate expressions for Euler angles in Wigner rotation.
• Insert alignment summation in to HelicityModel.
  docs: add TR-014 about amplitude sum notation compwa.github.io#112
• Move spin-alignment notebook to compwa-org (TR-015).
  docs: create TR-015 from AmpForm's spin-alignment compwa.github.io#119
• Convert spin-alignment notebook into pure usage notebook.
• Test procedure by reproducing distributions from Marangotto 2020 (Figure 2).
  docs: add TR-013 with spin alignment data test compwa.github.io#111
• Insert Subsystem alignment Wigner D functions into the helicity amplitude model #10
• Identify "intersection" of different Subsystems from two topologies #8

Additional ideas:

• Wrap Wigner rotation in UnevaluatedExpression class.

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