[ntuple] Merger: warn when merging RNTuples with higher versions than current

[silverweed]

This Pull request:

Makes the RNTupleMerger emit a warning when one of more RNTuples are merged that have a higher version number than the latest.

To do this I added a public RNTupleDescriptor::GetVersion() and a public static RNTuple::GetCurrentVersion() method that can be used to compare versions. I made them public because I think they can be useful for users in general, open for discussion.

TODO:

• add a way to prevent the merger from trying and merge such RNTuples (this would have to be both a RNTupleMergeOption field and a TFileMerger string option);
• add a test

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    22 files      22 suites   3d 4h 2m 55s ⏱️
 3 810 tests  3 808 ✅ 1 💤 1 ❌
75 850 runs  75 835 ✅ 9 💤 6 ❌

For more details on these failures, see this check.

Results for commit 4343adf.

♻️ This comment has been updated with latest results.

[jblomer]

LGTM