add interpolation option to append

@Marissa-Clark I think the best place to add the interpolation functionality we discussed is in the `append` or `concatenate` methods. We should have an option to interpolate voxel responses between the two images.

Look to to `stats.upsample` for an example using scipy interpolate methods.

`interpolate = interp1d(orig_spacing, data, kind=method)`

Start by locating the append and concatenate methods, then read stats.upsample for its use of scipy interpolation and the interp1d example in the issue. Clarify how the interpolation option should affect voxel responses between images, including the supported method values and expected behavior, before identifying validation coverage. Done means append or concatenate can perform the requested interpolation without changing its existing behavior by default.

Written by the indexing model from the issue text.