Export grid may fail if old sww files are around??

[ole]

In the presence for old sww files, test_all (verbose) gave the following error

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ERROR: Test that sww information can be converted correctly to asc/prj
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Traceback (most recent call last):
  File "shallow_water\test_data_manager.py", line 1345, in test_export_grid
    format = 'asc')
  File "X:\inundation\sandpits\onielsen\anuga_core\source\anuga\shallow_water\data_manager.py", line 1673, in export_grid
    format)
  File "X:\inundation\sandpits\onielsen\anuga_core\source\anuga\shallow_water\data_manager.py", line 1824, in sww2dem
    print '    t [s] in [%f, %f], len(t) == %d'\
ValueError: min() arg is an empty sequence

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ERROR: Test that sww information can be converted correctly to asc/prj
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Traceback (most recent call last):
  File "shallow_water\test_data_manager.py", line 1421, in test_export_gridII
    format = 'asc')
  File "X:\inundation\sandpits\onielsen\anuga_core\source\anuga\shallow_water\data_manager.py", line 1673, in export_grid
    format)
  File "X:\inundation\sandpits\onielsen\anuga_core\source\anuga\shallow_water\data_manager.py", line 1824, in sww2dem
    print '    t [s] in [%f, %f], len(t) == %d'\
ValueError: min() arg is an empty sequence

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ERROR: Test that sww information can be converted correctly to asc/prj
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Traceback (most recent call last):
  File "shallow_water\test_data_manager.py", line 1546, in test_export_gridIII
    format = 'asc')
  File "X:\inundation\sandpits\onielsen\anuga_core\source\anuga\shallow_water\data_manager.py", line 1673, in export_grid
    format)
  File "X:\inundation\sandpits\onielsen\anuga_core\source\anuga\shallow_water\data_manager.py", line 1824, in sww2dem
    print '    t [s] in [%f, %f], len(t) == %d'\
ValueError: min() arg is an empty sequence

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Ran 617 tests in 53.203s