Add tensorboard

[florian-huber]

So far only a simply writer for use with Tensorboard.

• tracks loss and rmse for train and val set
• could be expanded: e.g. by adding writer.add_graph to also track the model architecture

[niekdejonge]

I have not worked with tensorboard before so it is a bit hard to judge for me if this is implemented correctly. What I understand from the code is that some logging data will be stored in the folder "runs", which I assume can later be used to visualize part of the model in tensorboard.