"zipper tile" with DEM without same resolution between tiles

Algorithm for tiles with change in resolution between North/South edge
of tiles. No change of resolution taken into account between West/East
edge of tiles as known DEM does not have such change.
Algorithm done also for corner zipper tiles.
Test with dummy SRTM
To be done:
Test with real data from SRTM
1 job for zipper-tiles in 4 minutes and 17 seconds (queued for 3 seconds)