[df] First integration of `RHist` filling

[hahnjo]

As the classes are experimental, using the method will print a warning to inform the user.

Note that this is only a first version, there are a couple of advanced functionalities and better error handling that will come in future PRs.

[github-actions[bot]]

Test Results

    21 files      21 suites   3d 18h 43m 15s ⏱️  3 787 tests  3 787 ✅ 0 💤 0 ❌ 77 582 runs  77 582 ✅ 0 💤 0 ❌

Results for commit 46f531b6.