update/improve module docstrings + add copyright notices

This MR improves the docstring of aas.adapter._generic and updates the docstring of aas.model.aas. The docstring of aas.adapter._generic is still quite short, but I think it contains all information that is necessary to understand what the module does / what its use is.
So this closes issues #61 (closed) and #62 (closed).

Additionally this MR adds missing copyright notices to:

• aas.adapter._generic
• aas.model.concept
• test._helper.setup_testdb

aas/adapter/_generic.py

+# Copyright 2020 PyI40AAS Contributors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+# the License. You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations under the License.
 """
-Dicts to serialize and deserialize enum classes
+The dicts defined in this module are used in the json and xml modules to translate enum members of our
+implementation to the respective string and vice versa.
 """
 from typing import Dict