Linear interpolation might not be appropriate for the mask

[jcohenadad]

This code uses linear interpolation for the mask, which is good for metric extraction, but bad for figure generation.

https://github.com/spinal-cord-7t/coil-qc-code/blob/e6ac4b386a33ded3614fb6ccff742dd97e1c7330/data_processing-phantom.ipynb#L362-L364

two options:

• we change it to NN
• we binarize during figure generation