Implement general Chew-Mandelstam dispersion integral

[redeboer]

[Zeyna777]

shall i first extent to higher l with the dispersion integral or just the the analytic solution

[redeboer]

While writing this issue, I had in mind to create expression classes for the Chew–Mandelstam $S$ waves only like the ones you defined per-notebook in https://github.com/ComPWA/K-matrix-research.

There are already some functions in AmpForm, but from what I remember, they were either not correct or inconvenient to use.
https://ampform.readthedocs.io/0.15.x/api/ampform.dynamics.phasespace.html#ampform.dynamics.phasespace.chew_mandelstam_s_wave

These functions need to be cleaned up or corrected. Optionally, you can use the existing notebooks in K-matrix-research as a test:

pip install git+https://github.com/ComPWA/ampform@mycommit

or interactively, locally, assuming ampform is cloned next to K-matrix-research:

pip install -e ../ampform