MNT: Initial copy of Chris Rorden's gpu_test

gpu_test/ptest/masks/56.nii.gz

gpu_test/ptest/masks/57.nii.gz

gpu_test/ptest/masks/58.nii.gz

gpu_test/ptest/masks/59.nii.gz

gpu_test/ptest/masks/6.nii.gz

gpu_test/ptest/masks/60.nii.gz

gpu_test/ptest/masks/61.nii.gz

gpu_test/ptest/masks/62.nii.gz

gpu_test/ptest/masks/63.nii.gz

gpu_test/ptest/masks/64.nii.gz

gpu_test/ptest/masks/65.nii.gz

gpu_test/ptest/masks/66.nii.gz

gpu_test/ptest/masks/67.nii.gz

gpu_test/ptest/masks/68.nii.gz

gpu_test/ptest/masks/69.nii.gz

gpu_test/ptest/masks/7.nii.gz

gpu_test/ptest/masks/70.nii.gz

gpu_test/ptest/masks/71.nii.gz

gpu_test/ptest/masks/72.nii.gz

gpu_test/ptest/masks/73.nii.gz