[ntuple] Split RNTupleProcessor into RNTupleComposer and RNTupleProcessor

[enirolf]

Rationale

During the implementation of the RNTupleProcessor-based RDataSource, I realized that in this case, no processing is done by the RNTupleProcessor itself anymore, since this is already taken care of by RDF. From this point of view, I believe it makes sense to have a separation of concerns between the orchestration of the RNTuple compositions and the actual processor thereof, which is what this PR addresses.

Functionally, the changes are minimal. The bulk of the RNTupleProcessor class is moved to a new class, the RNTupleComposer, with only the iterator remaining. The RNTupleProcessor is now created by passing a reference to an RNTupleComposer.

The RNTupleComposer can now also serve as the backend for other data loading interfaces, most notably RDataSource as mentioned previously. In a similar vein, this could potentially open the possibility of merging the RNTupleProcessor into the RNTupleReader, but this requires further investigation.

Old interface

std::vector<RNTupleOpenSpec> ntuples = {{"ntuple1", "ntuple1.root"}, {"ntuple2", "ntuple2.root"}};
auto processor = RNTupleProcessor::CreateChain(ntuples);

auto pt = processor->RequestField<float>("pt");

for (const auto idx : *processor) {
   std::cout << "event = " << idx << ", pt = " << *pt << std::endl;
}

std::cout << "processed " << processor->GetNEntriesProcessed() << " events" << std::endl; 

New interface

std::vector<RNTupleOpenSpec> ntuples = {{"ntuple1", "ntuple1.root"}, {"ntuple2", "ntuple2.root"}};
auto composer = RNTupleComposer::CreateChain(ntuples);

auto pt = composer->RequestField<float>("pt");

RNTupleProcessor processor(*composer);

for (const auto idx : processor) {
   std::cout << "event = " << idx << ", pt = " << *pt << std::endl;
}

std::cout << "processed " << processor.GetNEntriesProcessed() << " events" << std::endl; 

[github-actions]

Test Results

    21 files      21 suites   3d 11h 34m 42s ⏱️
 3 867 tests  3 867 ✅ 0 💤 0 ❌
73 568 runs  73 568 ✅ 0 💤 0 ❌

Results for commit 8ced87e.