Merge from 5.34 commit:   f7eaf6ee1cb77b9e32a2b65360e59d8d36f2bcde

Fix TH1::Merge for merging histograms with buffer with histogram with limits
Add tests for merging histogram with buffer in stressHistogram and add tests for case with empty histogram
Merge from 5.34 patches:
commits :   32a4accb6016fee64cf92bba8068b95d320a7bca   and 7a7d2111f9785b8586e36b4309f6297aaa893392

Fix case of merging TProfile with different limits when the first profile is empty
(the fix applied previously for bug #5509 was not applied to the TProfile)

Improve also tests for 2D and 3D profile
Implement correctly the projections from a TProfile2D to TProfile and from a TProfile3D to a TProfile2D.
Add corresponding tests in stressHistogram.
This fixes bug ROOT-6220