Introduce machinery for uncorrelated NFs

Even when specifying NFs manually (using TQNFManualSetter), and even when specifying just a single NF, the NF uncertainties are propagated via the same machinery used in more complex NF algorithms, where correlations are estimated via toy experiments. This not only introduces a non-negligible extra compute time, but also slightly changes the NF and uncertainty from the values specified, which is particularly problematic when applying theory systematic uncertainties.

This update adds a new tag simpleScaleUncertainty for NFs set via manual setter that directs the machinery to propagate NF uncertainties as if they're uncorrelated, solving the aforementioned problems.