adding two clipped data sets fails

[sexton]

New functionality to clip a data set on an arbitrary polygon (either keep data in the polygon or outside of it). Get an error message when attempt to add two clipped data sets together (see no_test_clip1_inside_outside in test_geospatial_data.py)

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Traceback (most recent call last):

File "test_geospatial_data.py", line 496, in test_clip1_inside_outside

assert allclose((G1+G2).get_data_points(),points)

File "T:\sandpits\jsexton\ANUGA\anuga_core\source\anuga\geospatial_data\geospa

tial_data.py", line 408, in add

for x in self.attributes.keys():

AttributeError?: 'NoneType?' object has no attribute 'keys'

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[ole]

Fixed in changeset:3723