docs: def mgz_to_nifti(filename,prefix=None,gzip=True):
      '''Convert ``filename`` to a NIFTI file using ``mri_convert``'''
      setup_freesurfer()
      if prefix==None:
          prefix = nl.prefix(filename) + '.nii'
      if gzip and not prefix.endswith('.gz'):
          prefix += '.gz'
      nl.run([os.path.join(freesurfer_home,'bin','mri_convert'),filename,prefix],products=prefix)