[hist] Fix label merge when one axis is numerical and extendable

[lmoneta]

Create a new type of merging, a special case when the histogram has an axis with label and the other axis is numeric and different but compatible.

A new test is added also in stressHIstogram for this new case

This fixes #10828

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Build failed on windows10/cxx14. See console output.

[phsft-bot]

Build failed on ROOT-performance-centos8-multicore/cxx17. Running on olbdw-01.cern.ch:/data/sftnight/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.runtutorials.tutorial_roostats_rs601_HLFactoryexample

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Build failed on windows10/cxx14. Running on null:C:\build\workspace\root-pullrequests-build See console output.

Errors:

• [2022-08-01T15:53:45.975Z] LINK : fatal error LNK1104: cannot open file 'C:\build\workspace\root-pullrequests-build\build\bin\libCore.dll' [C:\build\workspace\root-pullrequests-build\build\core\Core.vcxproj]