New NIFTI_XFORM_ code under consideration

[effigies]

There is an open discussion on NITRC for a NIFTI_XFORM code 5 that will indicate alignment to an unspecified template. This use case has largely been relegated to NIFTI_XFORM_ALIGNED_ANAT (2) up to now.

I'm overall in support of this move, and it should be a very minimal effort to support these files. More importantly, if it happens, nibabel should be able to handle these files gracefully, so we should move quickly to adopt. This is particularly urgent in that we should have at least one Python 2-compatible release with this support, so that users on legacy operating systems that do not make Python 3 easy to get will be able to work with these files with a simple pip install.

Please contribute to the discussion on NITRC if you have any thoughts relevant to the standard, and here if you have any thoughts relevant to nibabel in particular.