Use pytorch-lightning to replace logging/monitoring and training/evaluation loops

Closely related to #21, we should investigate if the use of pytorch-lightning is not going to allow us to drop large parts of the code base presented in this package and biosignal/software/ptbench>.

In particular, the following features seem relevant:

• Training and evaluation loops: https://pytorch-lightning.readthedocs.io/en/latest/model/train_model_basic.html
• Logging to tensorboard compatible streams: https://pytorch-lightning.readthedocs.io/en/latest/visualize/logging_basic.html
• Early stopping: https://www.youtube.com/watch?v=vfB5Ax6ekHo
• Training on GPUs: https://www.youtube.com/watch?v=nsyVXthGRHo