Rate parameter malfunctions when there are also shape parameters

[JelleAalbers]

The implementation for rate parameters assumes the fraction of events that fall in the analysis space is the same for every model. This is not true when there are shape parameters.

A related problem exists for rate uncertainties (rate parameters with a prior). The prior is currently defined over the absolute rate, but this is affected by shape parameters too.

I'm going to switch rate parameters to be multipliers of the rate get out of this mess. This will mean a few examples in laidbax have to be changed.