Fix the unit information on the output

[ayllon]

The [] are not needed when going through CCfits (maybe it is for cfitsio?).
They end as-is on the output catalog, which confuses other FITS implementations like Astropy, which fails to identify the unit.
i.e

In [5]: t['world_centroid_alpha'].unit                                                                                                                                                        
Out[5]: UnrecognizedUnit([deg])

After this fix

In [12]: t['world_centroid_alpha'].unit                                                                                                                                                       
Out[12]: Unit("deg")