PGCompositeElement: Don't assign key attribute
Comparator: Set key attribute to None

This prevents the following error:

AttributeError: Neither 'PGCompositeElement' object nor 'Comparator'
object has an attribute 'key'
test_functional: Added ORM style test for ST_Dump()
types: Added GeometryDump type

This type can be used as the return type for the ST_Dump() function
functions: Added ST_Dump() function