TEST: Expand splinterpolator tests to cover all extrapolation methods, and

test/test_splinterpolator.cc

...plinterpolator/order_1_3d_extrap_constant_gradient.nii.gz

..._splinterpolator/order_1_3d_extrap_constant_values.nii.gz

..._splinterpolator/order_1_3d_extrap_mirror_gradient.nii.gz

...st_splinterpolator/order_1_3d_extrap_mirror_values.nii.gz

...plinterpolator/order_1_3d_extrap_periodic_gradient.nii.gz

..._splinterpolator/order_1_3d_extrap_periodic_values.nii.gz

...interpolator/order_1_3d_extrap_soft_zeros_gradient.nii.gz

...plinterpolator/order_1_3d_extrap_soft_zeros_values.nii.gz

...t_splinterpolator/order_1_3d_extrap_zeros_gradient.nii.gz

...est_splinterpolator/order_1_3d_extrap_zeros_values.nii.gz

...plinterpolator/order_2_3d_extrap_constant_gradient.nii.gz

..._splinterpolator/order_2_3d_extrap_constant_values.nii.gz

..._splinterpolator/order_2_3d_extrap_mirror_gradient.nii.gz

...st_splinterpolator/order_2_3d_extrap_mirror_values.nii.gz

...plinterpolator/order_2_3d_extrap_periodic_gradient.nii.gz

..._splinterpolator/order_2_3d_extrap_periodic_values.nii.gz

...interpolator/order_2_3d_extrap_soft_zeros_gradient.nii.gz

...plinterpolator/order_2_3d_extrap_soft_zeros_values.nii.gz

...t_splinterpolator/order_2_3d_extrap_zeros_gradient.nii.gz