feature: save model checkpoint and model output

[saraheisenach]

This PR adds in the ability to:

1. Save the model to a checkpoint file every epoch
2. Keep track of the best model during training and save it to a checkpoint file
3. Write the best model output to a csv file

The results will be saved like this:

In addition it fixes an issue with different models having different target outputs. Now, a model must specify what the target attribute is so that the trainer knows what attribute to use when writing to an output file.