Training fraction and end date allowed at the same time in config

[anders-kiaer]

Original authour: @wouterjdb

If you set both the training set fraction and end date you'll get the following warning when running FlowNet:

Training set fraction and training set end date are both defined in config file.
The input given for training set fraction will be ignored.
The training set end date will be used to calculate the training set fraction.

I'd actually suggest to let the config parser handle this and stop execution early on. A warning during execution is a bit 'too silent' if you ask me.