[RF] RooFit open issues and backports for 6.26.06 release

[guitargeek]

The usual issue to collect what is needed for RooFit in the next 6.26 patch release, this time for 6.26.06.

The related issues for the previous 6.26 releases:

1. [RF] RooFit open issues and backports for 6.26.00 release #9508
2. [RF] RooFit open issues and backports for 6.26.02 release #10112
3. [RF] RooFit open issues and backports for 6.26.04 release #10493

GitHub issues that still need to be addressed for the next patch release:

• [RF] HistFactory PreprocessFunction::PrintXML() needs to escape special characters to produce valid XML #10840
• [RF] sPlot does not work with RooAddPdf in 6.26/04 #10869

The bugfix PRs that need to be backported in this order to the 6.26 branch before the next 6.26 patch release:

• [RF] Implement binned fit optimization in BatchMode and other optimizations #9423
• [RF] Change buffer management in BatchMode such that queues get cleared #10736
• [RF] Fix wrong deletion of arg in workspace when using AddShapeFactor #10740
• [RF] Fix segmentation fault in BatchMode with multithreading #10754
• [RF] Implement likelihood offsetting in new RooFit BatchMode #10802
• [RF] Improve RooStats::HistFactory::PreprocessFunction class #10884
• [RF] Correctly sync proxy normalization sets in RooAddPdf::getValV() #10885
• [RF] RooFit::TestStatistics performance fixes #10832
• [RF] RooBarlowBeestonLL: Avoid removing elements of RooArgSet while looping over them  #10921
• [RF] Improvements to RooFit BatchMode for clean and correct computation graph of compiled likelihood #10909
• [RF] Cleanup RooLagrangianMorphFunc #9912
• [RF] Fixup to parameter index calculation in ParamHistFunc #10989
• [RF] Skip out-of-range events in RDataFrame to RooDataSet Helper #11018
• [RF] Enable streaming of RooLagrangianMorphFunc and other improvements #11023

The backports were grouped in a few PRs to v6-26-00-patches:

1. [v626][RF] Backports of RooFit PRs to v6-26-00-patches: Part 16 #10803
2. [v626][RF] Backports of RooFit PRs to v6-26-00-patches: Part 17 #10986
3. [v626][RF] Backports of RooFit PRs to v6-26-00-patches: Part 18 #11057