feat: add RNTuple with multiple cluster groups (#168)

* Added RNTuple with multiple cluster groups

* Small tweaks

dev/make-root/rntuple_multiple_cluster_groups.C

@@ -0,0 +1,34 @@
+/* https://root.cern/doc/master/ntpl001__staff_8C.html */
+/* this file tests when we have multiple pages, each page stores 65536 bytes,
+ * thus we need to make it longer */
+/* https://github.com/scikit-hep/uproot5/pull/630 */
+
+R__LOAD_LIBRARY(ROOTNTuple)
+#include <ROOT/RField.hxx>
+#include <ROOT/RNTuple.hxx>
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+#include <ROOT/RRawFile.hxx>
+
+using RNTupleModel = ROOT::Experimental::RNTupleModel;
+using RNTupleWriter = ROOT::Experimental::RNTupleWriter;
+
+void rntuple_multiple_cluster_groups() {
+  std::string rootFileName{"test_multiple_cluster_groups_rntuple_v1-0-0-0.root"};
+  auto model = RNTupleModel::Create();
+  auto int_field = model->MakeField<int>("one");
+  auto int_vector = model->MakeField<std::vector<int16_t>>("int_vector");
+  auto ntuple =
+      RNTupleWriter::Recreate(std::move(model), "ntuple", rootFileName);
+  for(auto i=0; i<1000; i++){
+      if (i && i%100==0) {
+          ntuple->CommitCluster();
+      }
+      if (i == 450 || i == 750) { // to add a bit of unevenness
+        ntuple->CommitCluster(/*commitClusterGroup*/ true);
+      }
+      *int_field = i;
+      *int_vector = {static_cast<int16_t>(i), static_cast<int16_t>(i+1)};
+      ntuple->Fill();
+  }
+}