Strip some characters from STC-S string definitions

[gbrammer]

Parse a string like

>>> import sregion
>>> sr = sregion.SRegion("""Union ICRS ( Polygon 239.807341 -18.296691 239.803564 -18.300277 239.799786 -18.296691 239.803563
-18.293105 Polygon 239.797826 -18.295944 239.794049 -18.299530 239.790272 -18.295944 239.794049
-18.292358)""")
>>> print(sr.xy)
[array([[239.807341, -18.296691],
        [239.803564, -18.300277],
        [239.799786, -18.296691],
        [239.803563, -18.293105]]),
 array([[239.797826, -18.295944],
        [239.794049, -18.29953 ],
        [239.790272, -18.295944],
        [239.794049, -18.292358]])]

The Union ICRS prefix is simply stripped from the string and the SRegion object is the combination of the two polygons. Nothing is done for sophisticated "region math" with regions, intersections, differences, etc.

[gbrammer]

The IVOA definition of the STC-S standard can be found at https://www.ivoa.net/documents/STC-S/20130917/WD-STC-S-1.0-20130917.html. SRegion makes no attempt at being fully compliant with this standard!