First argument of `FormFactor` should be in GeV, not GeV² (`s`)

[redeboer]

As mentioned by @mmikhasenko in RUB-EP1/amplitude-serialization#47 (comment), it often happens that you swap arguments in the FormFactor for production versus decay. It may be problematic if the caller expects to input mass for the first argument, just like for the other two arguments.

[redeboer]

@Zeyna777 this is relevant for your work in https://github.com/ComPWA/PWA-JPsi2pbarSigmaKS/issues/232 and #259

[redeboer]

I also wonder what is better: GeV² or GeV? Probably indeed GeV is better, because it removes the responsibility to take the square root from the FormFactor implementation.