Move ```compute_unc_histograms``` from user_custom to histTupleFlavor

[aebid]

Currently, the user_custom.yaml includes 3 keys about uncertainty handling.

compute_unc_variations: true
compute_unc_histograms: true
store_noncentral: true

I believe we should move the compute_unc_histograms: true into the histTuple_flavor logic of the global.yaml. https://github.com/cms-flaf/HH_bbWW/blob/b71186d006a6a8792317a9020a6adc9e8242e865/config/global.yaml#L407

This is because the histTuple_flavor usually dictates whether uncertainties are required or not. Usually uncertainties only need to be handled when we build histTuples for the limit setting (only the shape variable). In the current state, its very easy to forget to set this variable in the correct way and become confused.