[ntuple] Support creating ntuples within directories in RNTupleImporter

[siliataider]

This Pull request:

Changes or fixes:

This PR extends the functionality of the RNTupleImporter to allow creating ntuples within directories in the destination file during the conversion.

auto importer = RNTupleImporter::Create(treeFileName, "Tuple/DecayTree", ntupleFileName);

-> This would create the directory Tuple if it doesn't already exists in the destination file and place the ntuple DecayTree inside it.

Checklist:

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

This PR fixes #18965

[github-actions]

Test Results

    19 files      19 suites   3d 9h 43m 42s ⏱️
 2 813 tests  2 813 ✅ 0 💤 0 ❌
51 909 runs  51 909 ✅ 0 💤 0 ❌

Results for commit 351935f.

♻️ This comment has been updated with latest results.

[dpiparo]

thanks for these changes. Is it easy to add a test?

[dpiparo]

very nice. Thanks.

[silverweed]

Thanks! Maybe wait for @enirolf's approval since she's the author of the RNTupleImporter

[enirolf]

Not the author but I can still give my review :). Functionality looks good! I think it could benefit from some documentation but after that should be good to go.

[enirolf]

LGTM, thanks!