'Returns the value of the given string attribute node, None if the node
doesn\'t exist.  Can also take a tuple as a parameter, (target, child),
where child is the index of the attribute in the WKT.  For example:
>>> wkt = \'GEOGCS["WGS 84", DATUM["WGS_1984, ... AUTHORITY["EPSG","4326"]]\')
>>> srs = SpatialReference(wkt) # could also use \'WGS84\', or 4326
>>> print(srs[\'GEOGCS\'])
WGS 84
>>> print(srs[\'DATUM\'])
WGS_1984
>>> print(srs[\'AUTHORITY\'])
EPSG
>>> print(srs[\'AUTHORITY\', 1]) # The authority value
4326
>>> print(srs[\'TOWGS84\', 4]) # the fourth value in this wkt
0
>>> print(srs[\'UNIT|AUTHORITY\']) # For the units authority, have to use the pipe symbole.
EPSG
>>> print(srs[\'UNIT|AUTHORITY\', 1]) # The authority value for the untis
9122'

def __getitem__(self, target):

    if isinstance(target, tuple):
        return self.attr_value(*target)
    else:
        return self.attr_value(target)
