[hist] Implement initial RProfile

[hahnjo]

It builds on RHistEngine with an additional variable.

[silverweed]

Imho this should be called something more specific to histograms (RHistProfile? RProfileHist?), as "profile" is a generic term that could be overloaded (e.g. a timing profile) and since it's directly in the ROOT namespace we might want to avoid that confusion.

[hahnjo]

hm, I think the term "profile" is well established in HEP to not be confusing in the context of a data analysis framework. For example, historically the class is called TProfile, Boost.Histogram calls it make_profile, and for YODA it's BinnedProfile / Profile1D etc.

[silverweed]

I'm more concerned about ambiguities in internal usage by developers than about confusion from users (RProfile might be used as an internal class in some other context). Nothing that some namespaces can't solve of course, but might be worth thinking about.

[github-actions]

Test Results

    20 files      20 suites   3d 4h 10m 6s ⏱️
 3 867 tests  3 867 ✅ 0 💤 0 ❌
69 164 runs  69 164 ✅ 0 💤 0 ❌

Results for commit ce6f93e.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

❌ Patch coverage is 99.23077% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 59.77%. Comparing base (7548ce3) to head (ce6f93e).
⚠️ Report is 83 commits behind head on master.

Files with missing lines	Patch %	Lines
hist/histv7/inc/ROOT/RProfile.hxx	99.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #22588      +/-   ##
==========================================
+ Coverage   59.75%   59.77%   +0.02%     
==========================================
  Files        3367     3368       +1     
  Lines      494970   495515     +545     
  Branches    95206    95270      +64     
==========================================
+ Hits       295749   296187     +438     
- Misses     199221   199328     +107     

Flag	Coverage Δ
unittests	59.77% <99.23%> (+0.02%)	⬆️

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