Move non-essential outputs to 'debug' folder

[jkosciessa]

The pipeline currently produces many figures that may not be self-explanatory and can rather be considered temporary outputs for debugging. For example, t1_after_rotating_and_scaling shows whether the orientation of the MR image was successful.

It may be useful to tally which outputs could go into a debug output folder, and perhaps we would like to then also rename them to highlight the function they came from to quickly identify what went wrong. Documenting sanity checks would also be useful.