Load data as 'ANTsImage' class
Examples
if(interactive() && ants_available()) {
  ants <- load_ants()
  # Python string
  x1 <- ants$get_ants_data('r16')
  as_ANTsImage( x1 )
  # R character
  nii_path <- system.file(package = "RNifti",
                          "extdata", "example.nii.gz")
  as_ANTsImage( nii_path )
  # niftiImage object
  x2 <- RNifti::readNifti(nii_path)
  as_ANTsImage( x2 )
}