[hist] Implement RHistEngine::SetBinContent

[hahnjo]

This needs one indirection to construct the std::array of indices with the right (number of) elements.

[hahnjo]

After discussion with Jakob, we may decide that we don't want RHistEngine::SetBinContent in the public interface. We don't want it in RHist because the semantics are unclear together with global histogram statistics. That's not a problem for RHistEngine, but it is still questionable if users should call it. On the other hand, RHistEngine::SetBinContent is useful for testing, for example the conversion to TH*, but that we could also do with Internal functions that we befriend...

[github-actions]

Test Results

    22 files      22 suites   3d 19h 6m 58s ⏱️
 3 707 tests  3 707 ✅ 0 💤 0 ❌
79 579 runs  79 579 ✅ 0 💤 0 ❌

Results for commit f7271a8.

[hahnjo]

We decided to not add this method to the public interface because it has unclear semantics for a number of cases, in particular related to global histogram statistics. While it might be fine to allow it in RHistEngine, it would also be confusing to then not have it in RHist. #20392 removes the two mentions we had so far.