* Add support for multi-dimensional model input and making the x, y interface...

* Add support for multi-dimensional model input and making the x, y interface symmetrical on the model / training interface: (#707)

- x/y_transform extract x, y as numpy arrays or floats out of the record
  - x/y_translate convert the numpy arrays of floats into tf-readable dictionaries used in tf data.
* Simplify model interface by implementing output_types() directly in the base class using output_shapes() dictionary.
* Adding developer guide for own model development
* Updated donkey command documentation
* Improve asserts and type hints in keras.py
* Added missing __init__.py in parts module.
* Add cool ascii text for donkey init and update yml and setup files including mypy
* Remove model training test from Travis and change the test to relative convergence. This avoids random fall overs in CI.
* Added test of tf.data as used in the training pipeline through re-implementation of data transformation from tub records to tf expected dictionaries, for all currently supported models.