Integration of modular model-handling from MLAir

To enable a modular model handling of Keras models, the model class developed in MLAir will be integrated. More information on the model class can be obtained in the repository or in the accompanying paper](https://doi.org/10.5194/gmd-14-1553-2021).
With this issue, the DeepRU-architecture will also be introduced that has already been set-up in issue-branch #73.

changed the description

created branch michael_issue090-integration-mlair-model to address this issue

mentioned in issue #48 (closed)

mentioned in issue #93 (closed)

closed