MNT: Initial copy of Chris Rorden's gpu_test

gpu_test/ptest/masks/2.nii.gz

gpu_test/ptest/masks/20.nii.gz

gpu_test/ptest/masks/21.nii.gz

gpu_test/ptest/masks/22.nii.gz

gpu_test/ptest/masks/23.nii.gz

gpu_test/ptest/masks/24.nii.gz

gpu_test/ptest/masks/25.nii.gz

gpu_test/ptest/masks/26.nii.gz

gpu_test/ptest/masks/27.nii.gz

gpu_test/ptest/masks/28.nii.gz

gpu_test/ptest/masks/29.nii.gz

gpu_test/ptest/masks/3.nii.gz

gpu_test/ptest/masks/30.nii.gz

gpu_test/ptest/masks/31.nii.gz

gpu_test/ptest/masks/32.nii.gz

gpu_test/ptest/masks/33.nii.gz

gpu_test/ptest/masks/34.nii.gz

gpu_test/ptest/masks/35.nii.gz

gpu_test/ptest/masks/36.nii.gz

gpu_test/ptest/masks/37.nii.gz