Support specification of Nifti sform tolerance via environmental variable

Per discussion with @effigies , it would be helpful to enable specification of a higher nifti sform tolerance via environmental variable.

There are cases where nifti headers, perhaps edited manually, that may have unintentional small shear, it would be helpful to specify an override via environmental variable a tolerance for orthogonality.

This would be used in docker images that use FSL, ANTs, etc.

Possibly related?

- https://github.com/InsightSoftwareConsortium/ITK/blob/fb61d2cadc33bf50d1f0ec5b842b70a1a7021d0e/Modules/IO/NIFTI/src/itkNiftiImageIO.cxx#L1989-L2010

Todo:

- [ ] find a test case that demonstrates the issue
- [ ] create a patch (if needed) to enable specifying the related tolerance via environmental variable
- [ ] look into updated associated warnings to suggest setting the environmental variable if appropriate

CC @hjmjohnson