Automatic restarting

When restarting multiple times (for example, when training an expensive model or running on an HPC cluster with short time limits), it is useful to be able to train and restart multiple times with the same command.

In metatrain, this functionality is provided via the --continue auto (or -c auto) flag of mtt train. This flag will automatically restart the training from the last checkpoint, if one is found in the outputs/ of the current directory. If no checkpoint is found, the training will start from scratch.