Teach Vary to accept Report

[ikabadzhov]

Vary should support Report() RDF action, but currently does not. Minimal reproducer:

   auto h = ROOT::RDataFrame(10)
               .Define("x", [](ULong64_t e) { return int(e); }, {"rdfentry_"})
               .Filter([](int x) { return x > 5; }, {"x"}, "before")
               .Vary(
                  "x",
                  [](int x) {
                     return ROOT::RVecI{x - 1, x + 1};
                  },
                  {"x"}, 2)
               .Filter([](int x) { return x > 7; }, {"x"}, "after")
               .Report();
   ROOT::RDF::Experimental::VariationsFor(h);

CC: @eguiraud