adds nii2slices and nii2raslimits.
nii2slices is a generalized version of nii2png, with a pctile option to rescale data, and a colormap option that supports colormaps based on a csv list, on a range between two values, on an transparency-based scale, and on a json colormap file. nii2raslimits returns the limits in real world coordinates occupied by the volume. In pixel coordinates, this is spanned by the point (-0.5,-0.5,0.5) left-posterior-inferior corner and the (dims-1)+(0.5,0.5,0.5) right-anterior-superior corner. These values are transformed by the get_best_affine matrix to real world coordinates.