add AFBCP and update AFB

[peterstangl]

This pull request updates the calculation of the forward-backward asymmetry (AFB) and introduces a new observable for the CP asymmetry of the forward-backward asymmetry (AFBCP) in the flavio/physics/bdecays/bvll/observables.py module. The changes include the full expression of AFB in the presence of scalar operators, and extend the set of available observables.

Improvements to AFB calculation:

• The experimental and theoretical numerator functions for AFB (AFB_experiment_num and AFB_theory_num) now include both the '6s' and '6c' angular terms. Even though the '6c' vanishes in the absence of scalar operators or without lepton mass effects, it is now included for completeness.

Addition of new CP asymmetry observable:

• A new function AFBCP_experiment_num is added to compute the CP asymmetry of the forward-backward asymmetry, and the corresponding observable 'AFBCP' is registered in the observables dictionary, making this quantity available for analysis.

[Copilot]

Pull request overview

Updates the angular-observable layer for exclusive (B\to V\ell^+\ell^-) decays by extending the forward-backward asymmetry (A_\mathrm{FB}) to include the (J_{6c}) term (relevant with scalar operators / lepton-mass effects) and registering a new CP-odd observable for the forward-backward asymmetry, (A_\mathrm{FB}^\mathrm{CP}), in the bvll observable registry.

Changes:

• Extend AFB_experiment_num and AFB_theory_num to include both '6s' and '6c' angular terms.
• Add AFBCP_experiment_num based on the corresponding CP-asymmetric angular terms.
• Register 'AFBCP' in _observables so it is available as a standard observable/prediction.

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[Copilot]

Pull request overview

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