Document M0 use in reports

[tsalo]

Closes #324.

Changes proposed in this pull request

• Update workflow descriptions to describe which volume types are used to create reference images (for registration) and calibration images (for CBF calculation).
• Rename smoothkernel variable to smooth_kernel.
• Rename processing_target value controllabel to control so I can use it directly in workflow descriptions.
• Pin aslprep_build to 0.0.2.

[codecov-commenter]

Codecov Report

Attention: 11 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
aslprep/workflows/asl/base.py	87.28% <100.00%> (ø)
aslprep/workflows/asl/gecbf.py	92.17% <100.00%> (ø)
aslprep/workflows/asl/util.py	80.76% <ø> (ø)
aslprep/workflows/asl/ge_utils.py	87.23% <55.55%> (-7.51%)	⬇️
aslprep/workflows/asl/cbf.py	71.34% <22.22%> (-1.38%)	⬇️

... and 1 file with indirect coverage changes

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