[ntuple] Make RNTupleJoinProcessor composable

[enirolf]

This PR adds the possibility to compose RNTupleJoinProcessors from existing processor objects. Similar functionality is already in place for the RNTupleChainProcessor (see #17393), so with these additions the RNTupleProcessor is (almost*) fully composable.

*One caveat: the case where an auxiliary processor in the join is a join itself is not yet properly handled. This will be handled in a follow-up PR.

[github-actions]

Test Results

    18 files      18 suites   4d 9h 49m 34s ⏱️
 2 731 tests  2 731 ✅ 0 💤 0 ❌
47 722 runs  47 722 ✅ 0 💤 0 ❌

Results for commit affbe33.

♻️ This comment has been updated with latest results.

[hahnjo]

The four cleanup commits at the end are great! Some comments for consideration on the entry handling.

[pcanal]

*One caveat: the case where an auxiliary processor in the join is a join itself is not yet properly handled. This will be handled in a follow-up PR.

How does it fail? (hopefully 'loudly' :) ).

[enirolf]

*One caveat: the case where an auxiliary processor in the join is a join itself is not yet properly handled. This will be handled in a follow-up PR.

How does it fail? (hopefully 'loudly' :) ).

With an exception, is that loud enough :D? I've added a test for it as well:

root/tree/ntuple/test/ntuple_processor.cxx

Lines 375 to 391 in d7e3e59

	TEST_F(RNTupleProcessorTest, JoinedJoinComposedAuxiliary)
	{
	auto primaryProc = RNTupleProcessor::Create({fNTupleNames[0], fFileNames[0]});
	std::vector<std::unique_ptr<RNTupleProcessor>> auxProcsIntermediate;
	auxProcsIntermediate.emplace_back(RNTupleProcessor::Create({fNTupleNames[2], fFileNames[2]}, "ntuple_aux2"));
	std::vector<std::unique_ptr<RNTupleProcessor>> auxProcs;
	auxProcs.emplace_back(RNTupleProcessor::CreateJoin(RNTupleProcessor::Create({fNTupleNames[1], fFileNames[1]}),
	std::move(auxProcsIntermediate), {"i"}));
	try {
	auto proc = RNTupleProcessor::CreateJoin(std::move(primaryProc), std::move(auxProcs), {});
	} catch (const ROOT::RException &err) {
	EXPECT_THAT(err.what(), testing::HasSubstr("auxiliary RNTupleJoinProcessors are currently not supported"));
	}
	}

[vepadulano]

Thanks a lot! Some minor comments for now

[vepadulano]

Congrats for this last step towards making the RNTupleProcessor composable! Maybe consider simplifying the commit history before merging

[hahnjo]

Thanks for addressing my comments!