Allow additional callback arguments in Python API

[argilo]

The C API allows additional data to be passed into the callback function through an opaque pointer:

nrsc5/include/nrsc5.h

Lines 589 to 597 in a4b7883

	/**
	* Establish a callback function.
	*
	* @param[in] st pointer to an `nrsc5_t` session object
	* @param[in] callback pointer to an event handling function of two arguments
	* @param[in] opaque pointer to the function's intended 2nd argument
	*
	*/
	NRSC5_API void nrsc5_set_callback(nrsc5_t *st, nrsc5_callback_t callback, void *opaque);

But the Python API doesn't have such a mechanism. It's possible to use closures, but that's not always convenient. So I've added an optional callback_args which will be passed through to the callback function. A tuple or list of arguments can be supplied.