Enable the use of REWEIGHT_SPLITTING_PDF_SCALES

If this setting is set to 1, also the input scale for PDFs in reweighted shower
splittings and reweighted cluster splittings is affected by the factorisation
scale factor of a variation.

The MCATNLO one-step shower is an exception, it always applies the
factorisation (and renormalisation) scale factor during reweighting.