ftol for mincresample using input volume step sizes

When using mincresample, the tolerance/precision (ftol) for calculating the position of resampled voxels is based on the step sizes of the deformation grid. These step sizes can differ from the input volume being resampled. This commit (together with a commit in the minc-tools repository) base the ftol variable on the step sizes of the input volume rather than the deformation grid.