aasx: Allow writing individual objects

This MR adds the following functionality to AASXWriter:

• Write individual Identifiable objects (including multiple AAS and stand-alone Submodels, Assets, ConceptDescriptions) to a single AASX part: write_aas_objects()
• Control splitting of submodels when writing an AAS to an AASX package according to the part name conventions defined in DotAAS with write_aas(): new parameter submodel_split_parts

This fixes #94 (closed)

Additionally, I've added a new method get_identifier() to class Reference, which I used in the new implementation of the AASXWriter.write_aas() method.