enh(io): add a nifti image
Description
Nifti formats are now supported in sight. For unit test purposes, an additional nii image is required in the sight data
Proposal
Add a *.nii image in the data
Functional specifications
A unit test should be able to load from sight-data a *.nii image, well-formed and not empty
Technical specifications
Add a *.nii image in sight/image/nifti