Replace sct_maths with sct_separate_dmri_separate_dwi_and_b0 in tutorial dMRI preprocessing

[joshuacwnewton]

Checklist

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Description

This PR replaces sct_maths with sct_dmri_separate_dwi_and_b0 in batch_processing.sh and our tutorials. The reason for this is to get a mean image that uses only DWI images and excludes b0 images, which should make for a more reliable mask for cropping around the cord during motion correction.

(In reality, the change shouldn't be very noticeable at all (batch_processing.sh tests still passed), but this is still better instruction to give to those following our tutorials.)

Parallel changes were made in the tutorial dataset (spinalcordtoolbox/sct_tutorial_data#8) as well as the SCT Course slide deck.

Linked issues

Partially addresses #3614.

[mguaypaq]

Apart from one filename mention in an image, this change looks good:

• a text search for dmri_mean (without _dwi in the middle) under tutorials/diffusion-weighted-mri reveals no other instances to change;
• I followed the tutorial manually with the changes, and everything still works.

[codecov]

Codecov Report

Merging #3643 (5dd3bd3) into master (cbe2e48) will not change coverage.
The diff coverage is n/a.

Flag	Coverage Δ
api-tests	24.18% <ø> (ø)
cli-tests	56.66% <ø> (ø)

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