Change append_diag to use AffineError

Moving the parameters to a function allows me to use the same parameters
for testing the resample_to_output function (coming soon to a repo near
you).

Add:

* resampling images to images / mapped voxels
* resampling images to output space
* smoothing over voxel axes
* FWHM / sigma conversion

From review by Eric L.

Fix docstring giving wrong explanation for ND interpolation.

Allow user to specify a scalar meaning that voxel sizes for each
dimension of the image should be the same.

Allow rtol, atol for wrapper around ``allclose``.

Build some test images by resampling with SPM, and test our resampling
against SPM's resampling.