[Python][RDF] Enable Support for C++ Free Functions in RDF Define and Filter

[siliataider]

This Pull request:

This PR extends the RDataFrame pythonizations by adding support for passing C++ free functions to .Define() and Filter operations, with support for:

• Templated functions
• Overloaded functions
• Functions taking any input/output combination of
  • Fundamental types
  • ROOT types (e.g. TObject-derived classes)
  • User-defined C++ classes

The implementation works by converting the free function to std::function using the function's signature.

Warning

This currently doesn't work if the function takes templated types as input arguments like STL containers (e.g. std::vector<T>) because of #19358
Concretely, this doesn't work:

bool has_even_sum(const RVec<int>& v) {
   return Sum(v) % 2 == 0;
} 

Important

Prerequisites / TODO:

• Bring [PyROOT] Get names and types of all overloads' signature #9138 up to date and merge it: this implementation relies on the API provided by this PR for selecting the proper overload.
• cppyy.gbl.std.function(...) fails for template types like std::vector<int>, ROOT::RVec<int> #19358: this would unlock a wide range of use-cases, like the example mentioned above.
  - This should not block this PR though
• Update documentation / examples

Quick demo:

import ROOT

ROOT.gInterpreter.Declare(""" 
struct MyStruct {
    int a;
    MyStruct() : a(0) {}
    MyStruct(int a_) : a(a_) {}
};
                          
float struct_a(const MyStruct& s) {
    return s.a;
}                          

bool is_even(int x) {
    return x % 2 == 0;
}
                          
bool is_even(int x, float y) {
    return x % 2 == 0 && x == y;
}                                                 

template <typename T>
bool is_even_t(const T x, const float y) {
    return x % 2 == 0 && x == y;
}
""")

rdf = ROOT.RDataFrame(3) \
         .Define("x", "(int)rdfentry_") \
         .Define("y", "(float)rdfentry_") \
         .Define("is_even_x", ROOT.is_even, ["x"]) \
         .Define("is_even_x_y", ROOT.is_even, ["x", "y"]) \
         .Define("is_even_t", ROOT.is_even_t[int], ["x", "y"]) \
         .Define("my_struct", "MyStruct((int)rdfentry_)") \
         .Define("struct_a", ROOT.struct_a, ["my_struct"]) 
         
rdf.Display().Print()

[github-actions]

Test Results

    20 files      20 suites   3d 0h 3m 40s ⏱️
 3 219 tests  3 218 ✅ 0 💤 1 ❌
62 972 runs  62 971 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit e1c1f4c.

♻️ This comment has been updated with latest results.

[vepadulano]

Thank you, this is great! I left a couple of minor remarks

[vepadulano]

LGTM, very nicely done! I just left a comment to check commit consistency