Concatenate across directions as well as runs

[tsalo]

Closes none. This will primarily (only?) affect HCP data, which acquires fMRI runs with both run entities and dir entities. The acquisition order is counterbalanced, so we can't ensure that the data will be concatenated in the right order, but it should still be an improvement.

Changes proposed in this pull request

• Concatenate BOLD data across run and dir entities.
• Sort the files first by run, then by direction.
  • This is because HCP acquires run 1 in session 1 and run 2 in session 2. The order of directions (LR and RL) is counterbalanced, but run is consistent. Therefore we can't guarantee that LR --> RL is right (it'll be right for one run, but not the other), but we can guarantee that run 1 --> run 2.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
xcp_d/utils/bids.py	86.88% <100.00%> (+0.26%)	⬆️
xcp_d/workflows/base.py	97.12% <100.00%> (ø)
xcp_d/workflows/concatenation.py	87.23% <ø> (ø)

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