Enumerate chunks.
chunk_iter.RdReturns a data frame of (ix, iy, x_off, y_off, x_size, y_size, shape_id)
for every chunk. Offsets are 0-based. Sizes are clipped at the grid
edge: edge chunks are smaller, never padded to a uniform size.
shape_id classifies each chunk as "interior", "right", "bottom",
or "corner"; a regular chunk grid produces at most these four
distinct shapes.