Document M0 scaling

[tsalo]

Closes #293.

Changes proposed in this pull request

• Consistently refer to the M0 scale as m0_scale.
• Use appropriate scale values in tests, because most of them (if not all) don't seem to be scaled already.
  • For the GE tests, use a scale value of 96 (32 base scalar * 3 repetitions).
  • For the Siemens tests, use a value of 10.
  • I don't know the appropriate value for the Philips data.
• Remove reference to aslbids in usage page, since the package appears unmaintained and out-of-date.
• Note in usage page that BIDS expects ASL data to already be scaled.

Documentation that should be reviewed

Usage page.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 🎉

Comparison is base (a7ca3cc) 49.32% compared to head (7c29817) 49.34%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
+ Coverage   49.32%   49.34%   +0.01%     
==========================================
  Files         135      135              
  Lines       12265    12265              
==========================================
+ Hits         6050     6052       +2     
+ Misses       6215     6213       -2     

Impacted Files	Coverage Δ
aslprep/cli/parser.py	86.74% <ø> (ø)
aslprep/tests/test_cli.py	100.00% <ø> (ø)
aslprep/tests/test_interfaces_cbf_computation.py	100.00% <ø> (ø)
aslprep/interfaces/cbf_computation.py	80.96% <100.00%> (ø)
aslprep/workflows/asl/base.py	85.95% <100.00%> (ø)
aslprep/workflows/asl/cbf.py	81.45% <100.00%> (+0.50%)	⬆️
aslprep/workflows/asl/gecbf.py	97.24% <100.00%> (ø)
aslprep/workflows/base.py	92.68% <100.00%> (+1.98%)	⬆️

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