Refactor Backend concept for data persistence (eclipse-basyx#370)

Previously, we tried to use the `Backend` concept for both data persistence and data integration. 
This led to complicated cross-dependencies, architectures and badly maintainable code. 

Now, we decided to split these two concepts and use the existing `Backend` `ObjectStore`s for just the data persistence.

Concretely, this commit does the following:

1. Remove the abstract `Backend` class (backends.py), as well as their concrete implementations (`CouchDBBackend`, `LocalFileBackend`), as they are no longer needed
2. Remove the `Referable.source` attribute, as it was only needed for the `Backend` class
3. Remove methods related to the `Referable.source` attribute, such as `Referable.update()` and `Referable.commit()` (base.py), as they were used together to perform the data integration.
4. Adapt other related codes such as tutorials and tests to reflect these changes
5. Adapt documentation to reflect these changes

Author: s-heppner

sdk/README.md

@@ -127,7 +127,7 @@ For further examples and tutorials, check out the `basyx.aas.examples`-package.
 * [`tutorial_storage`](./basyx/aas/examples/tutorial_storage.py): Manage a larger number of Asset Administration Shells in an ObjectStore and resolve references
 * [`tutorial_serialization_deserialization`](./basyx/aas/examples/tutorial_serialization_deserialization.py): Use the JSON and XML serialization/deserialization for single objects or full standard-compliant files
 * [`tutorial_aasx`](./basyx/aas/examples/tutorial_aasx.py): Export Asset Administration Shells with related objects and auxiliary files to AASX package files
-* [`tutorial_backend_couchdb`](./basyx/aas/examples/tutorial_backend_couchdb.py): Use the *Backends* interface (`update()/commit()` methods) to manage and retrieve AAS objects in a CouchDB document database
+* [`tutorial_backend_couchdb`](./basyx/aas/examples/tutorial_backend_couchdb.py): Use the *CouchDBObjectStore* to manage and retrieve AAS objects in a CouchDB document database
 
 
 ### Documentation

sdk/basyx/aas/backend/__init__.py

@@ -1,7 +1,3 @@
 """
-This module implements a standardized way of integrating data from existing systems into AAS objects. To achieve this,
-the abstract :class:`~basyx.aas.backend.backends.Backend` class implements the class methods
-:meth:`~basyx.aas.backend.backends.Backend.update_object` and :meth:`~basyx.aas.backend.backends.Backend.commit_object`,
-which every implementation of a backend needs to overwrite. For a tutorial on how to implement a backend, see
-:ref:`this tutorial <tutorial_backend_couchdb>`
+This module implements a standardized way of persisting AAS objects using various backends.
 """