Revision 1803

All about making DataEmpty instances throw.
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Exposed getDim from AbstractDomain to python to fix bug.

Added isEmpty member to DataAbstract to allow it to throw is queries are 
made about a DataEmpty instance.


Added exceptions to DataAbstract, DataEmpty and Data to prevent calls 
being made against DataEmpty objects.
The following still work as expected on DataEmpty instances

copy, getDomain, getFunctionSpace, isEmpty, isExpanded, isProtected, 
isTagged, setprotection.

You can also call interpolate, however it should throw if you try to 
change FunctionSpaces.