Fix segmentation error when accessing pynifti data.
Fix segmentation error when accessing pynifti data.

Data was not copied, so it was only a reference to the underlying C-array.
When that C-array was freed, we had an invalid reference and would seg fault.

nipy/nireg,arokem/nipy,alexis-roche/nireg,alexis-roche/nipy,alexis-roche/niseg,alexis-roche/register,bthirion/nipy,arokem/nipy,alexis-roche/nipy,alexis-roche/register,arokem/nipy,bthirion/nipy,alexis-roche/register,alexis-roche/nipy,bthirion/nipy,alexis-roche/niseg,nipy/nireg,nipy/nipy-labs,alexis-roche/nireg,bthirion/nipy,arokem/nipy,alexis-roche/nipy,nipy/nipy-labs