Add more convenience methods to MockStream class

[adrn]

• Equivalent of the orbit align_circulation_with_z() to put progenitor at y=0, z=0 and rotate stream approximately into plane (Galactocentric) - added in Add new method MockStream.rotate_to_progenitor_plane() #510
• .get_stream_frame([prog_w]) (optional prog_w) to get a great circle coordinate system aligned with the stream
• get_stream_track() to extract the ridgeline