[RF] Allowing to use AddPreprocessFunction for shape factors in HistFactory

[TomasDado]

We would like to have an option to reparametrise shape-factors, i.e. the per-bin normalisation factors in HistFactory. This would be useful for template fits where different histograms represent different values of some parameter, e.g. mass. Currently, it seems that only NormFactors can be reparametrised and the code crashes with a seg fault when trying to reparametrise the shape factors. See the example attached.

shapeFactorCrash.zip

Describe the solution you'd like

We would like to be able to use HistFactory::Measurement::AddPreprocessFunction for shapefactors

Describe alternatives you've considered

There is a workaround by doing this reparametrisation manually, but it cumbersome and errorprone

Additional context

Note that even if this is not supported, the code should probably not crash with a seg fault (tested with ROOT Version: 6.27/01)