feat: update main to TRExFitter version 00-04-26
Updating to the latest tag https://gitlab.cern.ch/TRExStats/TRExFitter/-/tags/TRExFitter-00-04-26. This ports !225 (merged) to main
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To make this work, porting also the changes from !221 (merged) to pick up the StatAnalysis-provided version of xRooFit.
Edited by Alexander Held