Add test for attributes

structure/attributes/README.md

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+# Attributes
+
+## Fields
+
+ * `Int{32}`
+ * `Attr`
+
+one regular and one attribute field.
+
+## Entries
+
+1. Attribute set name
+2. Attribute set description
+3. List of attribute entries with value and regular field range
+
+The values can be found in the reference file `structure.attributes.json`.

structure/attributes/read.C

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+#include <ROOT/RNTupleReader.hxx>
+#include <ROOT/RVersion.hxx>
+
+#include <iostream>
+
+void read(std::string_view input = "structure.attributes.root",
+          std::string_view output = "structure.attributes.json") {
+#if ROOT_VERSION_CODE < ROOT_VERSION(6, 40, 0)
+  std::cout << "Skipped structure/attributes/read.C - ROOT version too old."
+            << std::endl;
+#else
+  // same logic as is read_structure.hxx, with additional special part for
+  // attributes
+  std::unique_ptr<ROOT::RNTupleReader> reader;
+  try {
+    reader = ROOT::RNTupleReader::Open("ntpl", input);
+  } catch (const ROOT::RException &e) {
+    std::ofstream os(std::string{output});
+    std::string msgWithoutStacktrace;
+    std::getline(std::istringstream(e.what()), msgWithoutStacktrace);
+    os << "{\n";
+    os << "  \"error\": \"" << msgWithoutStacktrace << "\"\n";
+    os << "}\n";
+    return;
+  } catch (const runtime_error &e) {
+    cout << "Skipped structure/attributes/read.C - " << e.what() << endl;
+    return;
+  }
+
+  std::ofstream os(std::string{output});
+  os << "[\n";
+  auto Int32 = 
+      reader->GetModel().GetDefaultEntry().GetPtr<std::int32_t>("Int32");
+  bool first = true;
+
+  auto attrSet = reader->OpenAttributeSet("Attributes");
+  auto attr = attrSet->GetModel().GetDefaultEntry().GetPtr<std::int32_t>("attr");
+
+  os << "  {\n";
+  os << "    \"Attribute set name\": \"" << attrSet->GetDescriptor().GetName() << "\"\n";
+  os << "  }, \n";
+  os << "  {\n";
+  os << "    \"Description\": \"" << attrSet->GetDescriptor().GetDescription() << "\"\n";
+  os << "  },\n";
+
+  // list of values for attr with its range
+  os << "  {\n";
+  os << "    \"Attr\": " << "[\n";
+  first = true;
+  for (auto attrIdx : attrSet->GetAttributes()) {
+    auto range = attrSet->LoadEntry(attrIdx);
+    os << "      {\n";
+    os << "        \"Value\": " << *attr << ",\n";
+    os << "        \"Range\": [" << *range.GetFirst() << ", " << *range.GetLast() << "]\n";
+    if (first) {
+      os << "      },\n";
+      first = false;
+    } else {
+      os << "      }\n";
+    }    
+    std::cout << "  attr = " << *attr << " (valid for range [" << *range.GetFirst() << ", " << *range.GetLast() << "])\n";
+  }
+  os << "    ]\n";
+  os << "  }";
+
+  if (!first) {
+    os << "\n";
+  }
+
+  os << "]\n";
+#endif
+}

structure/attributes/write.C

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+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+#include <ROOT/RVersion.hxx>
+
+#include <TFile.h>
+#include <cstdint>
+#include <string_view>
+
+void write(std::string_view filename = "structure.attributes.root") {
+#if ROOT_VERSION_CODE < ROOT_VERSION(6, 40, 0)
+  std::cout << "Skipped structure/attributes/write.C - ROOT version too old."
+            << std::endl;
+#else
+  auto model = ROOT::RNTupleModel::Create();
+  auto Int32 = model->MakeField<std::int32_t>("Int32");
+
+  auto file = std::unique_ptr<TFile>(TFile::Open(std::string(filename).c_str(), "RECREATE"));
+  auto writer = ROOT::RNTupleWriter::Append(std::move(model), "ntpl", *file);
+
+  // definition of attribute set with attribute fields
+  auto attrModel = ROOT::RNTupleModel::Create();
+  attrModel->SetDescription("Attribute set for testing");
+  auto attr = attrModel->MakeField<std::int32_t>("attr");
+  auto attrSet = writer->CreateAttributeSet(std::move(attrModel), "Attributes");
+
+  // first entry
+  auto attrRange = attrSet->BeginRange();
+  *Int32 = 1;
+  writer->Fill();
+  *attr = 1;
+  attrSet->CommitRange(std::move(attrRange));
+
+  // second and third entry
+  attrRange = attrSet->BeginRange();
+  *attr = 2;
+  *Int32 = 2;
+  writer->Fill();
+  *Int32 = 3;
+  writer->Fill();
+  attrSet->CommitRange(std::move(attrRange));
+#endif
+}