[RF] RooFit open issues and backports for 6.26.12 release

[guitargeek]

The usual issue to collect what is needed for RooFit in the next 6.26 patch release, this time for 6.26.10.

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
4. [RF] RooFit open issues and backports for 6.26.06 release #10758
5. [RF] RooFit open issues and backports for 6.26.08/10 releases #11059

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

• [RF] RooFormulaVar not working in Root 6.26 #11760

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

• [RF] Fix wrong order of iterating over curves in RooPlot::residHist()  #11566
• [RF] Remove error in RooWorkspace::getSnapshot() if no snapshot found #11584
  Only the first commit that is not code modernization
• [RF] Fix normalization issues with RooGenericPdf and RooFormulaVar #10873
• [RF] Re-implement multi-range fits #11455
  Only the third commit that fixes a problem with the RooBinSamplingPdf, backporting the full multi-range fit implementation is way too dangerous.
• [RF] Disable the BatchMode usage in RooBinIntegrator for now #11912
  Only the second and third commit about the RooBinIntegrator
• [RF] Fix signature of RooWorkspace::getSnapshots() #12119
• [RF] Support RooAbsCategoryLValue::setBin() for non-existing ranges  #12305
  Only the first commit that is not code modernization.
• [RF] Don't throw an error if roobatchcompute lib was already loaded #12696
• [RF] Fix off-by-one error in HistoToWorkspaceFactoryFast #12707
• v626 HistFactory: open channel and combined files only once #12867
• [RF][HF] Add missing LinkDefs for systematics classes in HistFactory #12969
• [RF][HF] Remove commented-out Systematic class from HistFactory #12973

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 24 #11748
2. [v626][RF] Backports of RooFit PRs to v6-26-00-patches: Part 25 #11848
3. [v626][RF] Backports of RooFit PRs to v6-26-00-patches: Part 26 #11961
4. [v626][RF] Backports of RooFit PRs to v6-26-00-patches: Part 27 #12322
5. [v626][RF] Backports of RooFit PRs to v6-26-00-patches: Part 28 #12709
6. [v626][RF][HF] Add missing LinkDefs for systematics classes in HistFactory #13728
7. [v626][RF][HF] Remove commented-out Systematic class from HistFactory #13773

[guitargeek]

Cosing since there are currently no open problems or PRs to be backported anymore.