This adds 'python-dateutil>=2.8' as new runtime dependency.

Currently not working:
* correct handling of date types
* values in any other object than Proprerty
* example data / tests

Tests for the datatype functionality are also missing.

test._helper, adapter.json: add test for example_aas_missing_attributes and fix view deserialisation in json

Add couchdb backed ObjectStore

See merge request acplt/pyaas!3

The new class IEC61360ConceptDescription replaces ConceptDescription
with IEC61360DataSpecification template as data_specification.

We didn't support HasDataSpecification with its semantic for adding dynamic
attributes yet. After some discussion, we came to the conclusion, that
the concept of HasDataSpecification has many flaws, as it does not allow
a type-safe and deterministic implementation. It may not be
implementable in other programming languates without severe workarounds
at all. Instead, all use cases for dataSpecifications can easily handled
with normal class inheritence.

Thus, we won't support HasDataSpcification in PyI40AAS and want to work
towards removing HasDataSpecification from the AAS standard.

This includes adding the helper script test/_helper/setup_testdb.py for
configuring the CouchDB server.

This is a preperation for allowing simple subclassing of the decoder
with replacing constructed object types.