After computing per-scene pXpYD arrays, saves a tile-resolution
hyperstack <ts>-NADIR-DISPARITY-MAPS.tiff with:
  - frames = scenes (labelled by timestamp)
  - slices = pX, pY, D (3 channels)
  - NaN where scene has no pose or tile has no data

Useful for verifying disparity transform coverage before feeding
the rendered images into OpenMVS/COLMAP.
Co-authored-by: Claude <claude@elphel.com>