Add AASX reading and writing functionality

This MR adds the adapter.aasx module with AASXReader and AASXWriter classes to read and write .aasx files according to DotAAS section 7. Fixes #38 (closed).

It also adds/changes some core functionality to make this possible (or make testing easier):

• New module aas.util.traversal for traversing AAS object structures
• Improved DictObjectStore.__init__()
• Fixing paths in test.compliance_tool for unixoid systems
• Add Submodel references for identification and BOM submodels to Test_AssetAdministrationShell in examples.data.example_aas