fix to support templated callbacks

[Vipul-Cariappa]

TODO:

• Add new test for this change

The following should be sufficient:

import cppyy

cppyy.cppdef(
    r"""
#include <utility>
#include <iostream>
#include <string>

void foo() { std::cout << "foo!\n";}

void bar(int a, float b) { std::cout << "bar(" << a << "," << b << ")\n"; }

template <typename T, typename U>
void baz(T a, U b, std::string c) { std::cout << "baz(" << a << "," << b << ",\"" << c << "\")\n"; }

template<typename F, typename... Args>
void dataframe_define_mock(F callable, Args&&... args) {
    callable(std::forward<Args>(args)...);
}
"""
)

cppyy.gbl.dataframe_define_mock(cppyy.gbl.foo)
cppyy.gbl.dataframe_define_mock(cppyy.gbl.bar, 42, 11.11)
cppyy.gbl.dataframe_define_mock(cppyy.gbl.baz[int, float], 33, 101.101, "hello")

[vgvassilev]

LGTM!