Improve train/test split

[mastoffel]

There's a lot to improve in our training / test set split in data_splitting.py:

• the data is always split. However, we would want test_set_size=0 to work, so there's no split and AutoEmulate uses the full data as training data.
• data is randomly shuffled before being split. For temporal data, grouped data etc. we want more sophisticated splitting techniques.