Add indication of lowest validation loss epochs on loss plots

Closes #70

See merge request biosignal/software/mednet!39

Replace sampler balancing by loss balancing

Closes #6

See merge request biosignal/software/mednet!38

During validation logging, lightning seems to append "/dataloader_idx_n"
to the key we define if multiple dataloaders are used.

This currently only supports a single validation dataloader

Add dockerfile

See merge request biosignal/software/mednet!37