[ntuple] use namespace ROOT::Experimental in RNTupleDescriptor.cxx

[silverweed]

precursor of https://github.com/root-project/root/pull/16166

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Test Results

    13 files      13 suites   2d 21h 34m 18s :stopwatch:  3 025 tests  3 025 :white_check_mark: 0 :zzz: 0 :x: 33 786 runs  33 786 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 3864456e.

[enirolf]

IMO if we change it here, we should be consistent and change it everywhere and I'm not sure if that is worth the effort. Using the namespace might make it slightly easier to move out of Experimental but a search and replace all also wouldn't take much effort..

[silverweed]

The move out of Experimental is not really a driving motivation for this change (I hadn't even considered that), but making the file more information-dense is. Maybe it's only me, but I find it akin to obfuscation to have half of the page covered in namespaces and it makes the code harder to read for no real benefit (in this case).

I agree that we should be consistent with it, but I think it can be a gradual process as there is no practical harm in having an inconsistent state for some time (less harm than, say, having some of the files not being properly clang-formatted, which is a real waste of time in some cases).

That said, if the majority thinks it's not worth changing, I'll just close this PR and change it back in the other related PR.

[jblomer]

As discussed, let's close it for the time being. The situation will look different with RNTuple classes moving out of experimental.