Fix subplot args convert

[Sologala]

Unproperly args converting in void subplot(long , long , long) may cause segment fault.

matplotlib-cpp/matplotlibcpp.h

Lines 2252 to 2267 in 2acb4d0

	inline void subplot(long nrows, long ncols, long plot_number)
	{
	detail::_interpreter::get();
	// construct positional args
	PyObject* args = PyTuple_New(3);
	PyTuple_SetItem(args, 0, PyLong_FromLong(nrows));
	PyTuple_SetItem(args, 1, PyLong_FromLong(ncols));
	PyTuple_SetItem(args, 2, PyLong_FromLong(plot_number));
	PyObject* res = PyObject_CallObject(detail::_interpreter::get().s_python_function_subplot, args);
	if(!res) throw std::runtime_error("Call to subplot() failed.");
	Py_DECREF(args);
	Py_DECREF(res);
	}

[SirTwinkleberry]

The implicit conversion part is a duplicate of #333

[ArterioRodrigues]

Would double conversion also cause a seg fault? I am using this it seems to work.

 PyObject* args = PyTuple_New(3); 
     PyTuple_SetItem(args, 0, PyDouble_FromLong(nrows)); 
     PyTuple_SetItem(args, 1, PyDouble_FromLong(ncols)); 
     PyTuple_SetItem(args, 2, PyDouble_FromLong(plot_number));