feat: Extract image orientation from NIfTI header

[effigies]

For #76 and #79, it would be helpful to have access to the image orientation. This PR starts with a function to determine axis codes from the the image affine.

If we add nifti_header.axis_codes to the context, we could then write #76 as:

NiftiPEDir:
  issue:
    code: NIFTI_PE_DIRECTION_CONSISTENCY
    message: Phase encoding direction is inconsistent with entity
    level: warning
  selectors:
    - type(nifti_header) != "null"
    - intersects([entities.direction], ["AP", "PA", "RL", "LR", "IS", "SI"])
    - sidecar.PhaseEncodingDirection
  checks:
    - |
      entities.direction[2 - length(sidecar.PhaseEncodingDirection)]
      == nifti_header.axis_codes[index(["i", "j", "k"], sidecar.PhaseEncodingDirection[0]]

(Would need to check that check... It's probably being too clever, but it is nice to get it in one.)