This function checks if a function name or method name is available in the global namespace or the desired package namespace
get_existence_of_fun(function_name, package)
(character
) The name of the function to look up
(character
) The name of the package where this function
might be hidden (not exported)
"global" if it is available, "package" if it's available within the package or an Error if it is not available at all.