fix: Nifti terms were replaced by "image/nifti"

As discussed in https://github.com/incf-nidash/nidm/pull/158
fix: Nifti terms were replaced by "image/nifti"

As discussed in https://github.com/incf-nidash/nidm/pull/158
fix: Inf -> INF
fix: small inacurracies
Merge pull request #219 from cmaumet/fix_spm_gt

Small fixes in SPM ground truth