do not let mesh interface accept duplicate tags

[duncan]

Do not let mesh interface accept duplicate tags. See revision 3133 for a great test Ole made to show this feature

additionally, catch bad indexes of segments. this is what currently happens;

Traceback (most recent call last):

  File "run_sydney_smf.py", line 146, in ?

    verbose = True)

  File "Z:\inundation\sandpits\jsexton\ANUGA\inundation\caching\caching.py", lin

e 349, in cache

    T = apply(func,args,kwargs)

  File "Z:\inundation\sandpits\jsexton\ANUGA\inundation\pmesh\mesh_interface.py"

, line 86, in create_mesh_from_regions

    geo_reference=poly_geo_reference)

  File "Z:\inundation\sandpits\jsexton\ANUGA\inundation\pmesh\mesh.py", line 660

, in add_region_from_polygon

    region=create_region)

  File "Z:\inundation\sandpits\jsexton\ANUGA\inundation\pmesh\mesh.py", line 719

, in _add_area_from_polygon

    segment_tags[i] = key

IndexError: list assignment index out of range