Add recentre_on_sinks method to SarracenDataFrame class definition

[vasu-prasad]

Added a method to sarracen_dataframe.py that moves the data's coordinate origin to a specified sink particle for multi-sink datasets.

[ttricco]

I'm going to close this PR based on our discussed this week at 2nd European Phantom Workshop.

If re-centreing all particles around a sink, here is an minimal example of how it could be done (for sink 0):

sdf[['x', 'y', 'z']] += sdf_sinks[['x', 'y', 'z']].loc[0]