BUG: Serialize a geometry Parameter with aggregate="true", multiple="true".

Better identification of parameters with multiple components is performed in
serialization/deserialization by using GetMultiple().

This handles the exceptional case where the multiple attribute is "true" but
an aggregate attribute is also "true".  This case is added to the test suite.
It comes form the Slicer ModelMaker CLI.
Merge pull request #29 from thewtex/serialize-geometry-argument

BUG: Serialize a geometry Parameter with aggregate="true", multiple="tru...