upgrade code to TensorFlow V2

Following https://www.tensorflow.org/guide/migrate/upgrade I will analyze what is needed to upgrade to TensorFlow 2. Probably most effort will run into converting keras.* functions into tf.keras.* mode, but this is only a first guess.

testing status

• local machine @weichselbaum1 -> works on Ubuntu 20.04.3 LTS with Python 3.8.5 (after tabulate and pydot added to requirements.txt), graphviz installed manually
• local machine @kleinert1
• local machine @leufen1 -> pipeline finished successfully: !350 (merged)
• JUWELS @weichselbaum1
• HDFML @leufen1
• zam347 @leufen1 -> tested installation + python run.py
• CI test pipeline @leufen1