BUG: Only create tiles from visible data

If _chip_insertions slices returns data_slice = None, the chip
lies entirely outside the output array. We don't need to consider
it anymore since it will never touch the output plane.

Resolves #16

lentil/prop.py

@@ -122,7 +122,8 @@ def propagate(planes, wave, weight=None, npix=None, npix_chip=None, oversample=2
                     data_slice, chip_slice = _chip_insertion_slices(output_shape,
                                                                     (w.data[d].shape[0], w.data[d].shape[1]),
                                                                     shift)
-                    tiles.append(imtile(w.data[d], data_slice, chip_slice))
+                    if data_slice:
+                        tiles.append(imtile(w.data[d], data_slice, chip_slice))
 
                 tiles = consolidate(tiles)