[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.