Follow-up for tensorboard statistics + dnadna resume training

!36 (merged) adds support for outputting error statistics during training using the tensorboard format, as well as saving model checkpoints. This needs some follow-up work, some of which might be further split into separate issues:

• The tensorboard format is useful and a de facto "standard", but not very user-friendly except when using tensorboard. It would be good if when a training is completed, or better yet at a regular checkpoint, that the tensorboard statistics were also written out to a basic CSV file.

• Add tests for the error statistics output.

• Model checkpoints are saved, but the code is not yet implemented for resuming training from a checkpoint.

  • This should also have tests.