ENH: Add support for ASL data

[tsalo]

Closes none, but is related to #405. I'm happy to take a crack at adding tests, but it would be good to know what the preferred approach is. The easiest thing might be for me to just copy existing test files and rename them. I don't think distortion correction of EPI ASL will be any different from SDC of other EPI sequences, but I could add actual ASL and M0 data if that would be better. Unfortunately, I don't have access to data from a number of the cases listed below.

There are a few cases that I think should be covered. Do they make sense?

1. ASL scan with AP and PA M0 scans located in fmap, with separate M0 scan in perf.
2. AP ASL scan with PA M0 scan in fmap, with separate M0 scan in perf.
3. AP and PA ASL scans (is that supported yet?), with separate M0 scan in perf.
4. ASL scan with phase-difference field maps, with separate M0 scan in perf.
5. ASL scan without any field map.

Changes proposed:

• Add asl and m0scan to PEPOLAR and fieldmap-less suffix lists.
  • I essentially added "asl" wherever I saw "dwi".

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (53482c1) 80.59% compared to head (3ef588a) 80.59%.

Additional details and impacted files

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