included when the test data was first added

constant/zeros to previous incorrect method, to preserve existing behaviour

ownership of data pointer

order=2 interpolation

code was added in an early commit, but was then replaced with indx2indx, and
never updated / used.

to zero, but still interpolates out-of-bounds, to allow a smooth transition
from in-to-out of bounds

existing set of spline coefficients

more robust, pre-existing niftio routine