Use tmp folders in the training test

Closes #49 (closed)

The configurations now expects the full path to the output_folder instead of its name. This introduces changes in both training configs:

• document -> https://gitlab.com/teklia/atr/dan/-/blob/c82aaed01c02b77a000d429ed5020c48a5fef04f/dan/ocr/document/train.py#L180
• line -> https://gitlab.com/teklia/atr/dan/-/blob/c82aaed01c02b77a000d429ed5020c48a5fef04f/dan/ocr/line/train.py#L151