Feature/backend couchdb

Closes #78

See merge request !50

This does not resolve the RessourceWarning issued after using the
CouchDBBackend, but it still seems to be a good idea to close file descriptors
after usage.

compliance_tool: add aasx functionality

Closes #81 and #68

See merge request !53

adapter.json: support serializing/deserializing stripped objects

Closes #63

See merge request !61

test.adapter.xml: improve xml deserialization tests

See merge request !62

Fix AASX double relationship part writes

Closes #107

See merge request !63

This was done to avoid passing the  parameter to every encoder function manually.

don't import top-level modules relative