[df] Improve syntax for column definition in df001

The tutorial uses a C++ lambda with increment of a shared local variable between
two different nodes of the computation graph, without explaining it. This is a
somewhat advanced usage of the API for an introductory tutorial and is also in a
seemingly irrelevant part of the tutorial (used just for creating the toy data).
RDataFrame already provides the `rdfentry_` magic column which does effectively
the same, providing a sequentially increasing number. Also, it was already used
in the Python tutorial so this commit is just aligning the two language
versions.

tutorials/dataframe/df001_introduction.C

@@ -20,14 +20,8 @@
 void fill_tree(const char *treeName, const char *fileName)
 {
    ROOT::RDataFrame d(10);
-   int i(0);
-   d.Define("b1", [&i]() { return (double)i; })
-      .Define("b2",
-              [&i]() {
-                 auto j = i * i;
-                 ++i;
-                 return j;
-              })
+   d.Define("b1", [](ULong64_t entry) -> double { return entry; }, {"rdfentry_"})
+      .Define("b2", [](ULong64_t entry) -> int { return entry * entry; }, {"rdfentry_"})
       .Snapshot(treeName, fileName);
 }
 

tutorials/dataframe/df001_introduction.py

@@ -15,11 +15,11 @@
 
 import ROOT
 
-# A simple helper function to fill a test tree: this makes the example stand-alone.
 def fill_tree(treeName, fileName):
+    """A simple helper function to fill a test tree: this makes the example stand-alone."""
     df = ROOT.RDataFrame(10)
-    df.Define("b1", "(double) rdfentry_")\
-      .Define("b2", "(int) rdfentry_ * rdfentry_").Snapshot(treeName, fileName)
+    df.Define("b1", "static_cast<double>(rdfentry_)")\
+      .Define("b2", "static_cast<int>(rdfentry_ * rdfentry_)").Snapshot(treeName, fileName)
 
 # We prepare an input tree to run on
 fileName = "df001_introduction_py.root"