Cast result from mapinfo.data to bytes before passing to x-lal-based from_buffer() methods

This merge request addresses #75 (closed) when running on newer versions of GStreamer (1.18.x). Can confirm that there's no appreciable performance hit in older versions of GStreamer, but allowing these x-lal-based containers to support the buffer protocol may improve performance in newer versions as this could potentially allow reading buffers from the python layer without having to do a copy.

Closes #75 (closed).