[df] Improve naming of column to bitmask mapping

The object saved together with the output dataset by Snapshot with variations now features `column` rather than `branch` in the name, to be more generic. We also avoid mixing snake_case with CamelCase in the name, opting for the first.

Author: vepadulano

tree/dataframe/src/RDFSnapshotHelpers.cxx

@@ -1035,7 +1035,7 @@ struct SnapshotOutputWriter {
    {
       if (!fBranchToBitmaskMapping.empty()) {
          fFile->WriteObject(&fBranchToBitmaskMapping,
-                            (std::string{"R_rdf_branchToBitmaskMapping_"} + fTree->GetName()).c_str());
+                            (std::string{"R_rdf_column_to_bitmask_mapping_"} + fTree->GetName()).c_str());
       }
       if (fTree) {
          // use AutoSave to flush TTree contents because TTree::Write writes in gDirectory, not in fDirectory

tree/dataframe/src/RDataFrame.cxx

@@ -1270,10 +1270,10 @@ dataset as a `std::uin64_t`. For every 64 columns, a new bitmask column is added
 
 Each column that might contain invalid values is connected to exactly one bit in one bitmask. A mapping of column names
 to the corresponding bitmask is placed in the same file as the output dataset, with a name that follows the pattern
-`"R_rdf_branchToBitmaskMapping_<NAME_OF_THE_DATASET>"`. It is of type
+`"R_rdf_column_to_bitmask_mapping_<NAME_OF_THE_DATASET>"`. It is of type
 `std::unordered_map<std::string, std::pair<std::string, unsigned int>>`, and maps a column name to the name of the
 bitmask column and the index of the relevant bit. For example, in the same file as the dataset "Events" there would be
-an object named `R_rdf_branchToBitmaskMapping_Events`. This object for example would describe a connection such as:
+an object named `R_rdf_column_to_bitmask_mapping_Events`. This object for example would describe a connection such as:
 
 ~~~
 muon_pt --> (R_rdf_mask_Events_0, 42)

tree/dataframe/src/RTTreeDS.cxx

@@ -522,7 +522,7 @@ ROOT::Internal::RDF::RTTreeDS::CreateColumnReader(unsigned int /*slot*/, std::st
    if (TDirectory *treeDir = treeReader->GetTree()->GetDirectory(); treeDir) {
       using Map_t = std::unordered_map<std::string, std::pair<std::string, unsigned int>>;
       const std::string bitmaskMapName =
-         std::string{"R_rdf_branchToBitmaskMapping_"} + treeReader->GetTree()->GetName();
+         std::string{"R_rdf_column_to_bitmask_mapping_"} + treeReader->GetTree()->GetName();
       if (Map_t const *columnMaskMap = treeDir->Get<Map_t>(bitmaskMapName.c_str()); columnMaskMap) {
          if (auto it = columnMaskMap->find(std::string(col)); it != columnMaskMap->end()) {
             colReader = std::make_unique<RMaskedColumnReader>(*treeReader, std::move(colReader), it->second.first,

tree/dataframe/test/dataframe_snapshotWithVariations.cxx

@@ -219,7 +219,7 @@ TEST(RDFVarySnapshot, Bitmask)
       ASSERT_NE(branch, nullptr);
 
       auto *branchToIndexMap = file.Get<std::unordered_map<std::string, std::pair<std::string, unsigned int>>>(
-         ("R_rdf_branchToBitmaskMapping_" + treename).c_str());
+         ("R_rdf_column_to_bitmask_mapping_" + treename).c_str());
       ASSERT_NE(branchToIndexMap, nullptr);
       for (const auto branchName : {"x", "y", "x__xVar_0", "x__xVar_1", "y__xVar_0", "y__xVar_0"}) {
          ASSERT_NE(branchToIndexMap->find(branchName), branchToIndexMap->end());
@@ -339,8 +339,8 @@ TEST(RDFVarySnapshot, TwoVariationsInSameFile)
    auto snap2 = rdf.Filter(cuts2, {"x", "y"}).Snapshot(treename2, filename, {"x", "y"}, options);
 
    std::unique_ptr<TFile> file{TFile::Open(filename)};
-   EXPECT_NE(file->GetKey(("R_rdf_branchToBitmaskMapping_" + treename1).c_str()), nullptr);
-   EXPECT_NE(file->GetKey(("R_rdf_branchToBitmaskMapping_" + treename2).c_str()), nullptr);
+   EXPECT_NE(file->GetKey(("R_rdf_column_to_bitmask_mapping_" + treename1).c_str()), nullptr);
+   EXPECT_NE(file->GetKey(("R_rdf_column_to_bitmask_mapping_" + treename2).c_str()), nullptr);
 
    // In Windows, an exception is thrown as expected, but it cannot be caught for the time being:
 #if !defined(_MSC_VER) || defined(R__ENABLE_BROKEN_WIN_TESTS)