Feature request: systematics variation as a function of POI for template models

[anigamova]

Requested twice at least: https://cms-talk.web.cern.ch/t/parametrization-of-shape-systematics-vs-poi/138817 and https://cms-talk.web.cern.ch/t/using-higgscombine-to-measure-top-yukawa-coupling-values/22541/8