Custom DEM about s1prepro

[benjimin]

This is now resolved, and ocean is now visible.

Had to implement subsetting (as otherwise runtime increased to ~1hr10min/scene). Unfortunately, runtime still increased from ~10min/scene toward ~20min/scene.

A more efficient implementation would be conceivable (e.g. subsetting should not impact performance, and terrain flattening and ortho should be a single step). Also, since the satellite is constrained to about a 50m tube, should not need to fully recompute layover masks either.

The (hereby obsolete) version with auto-download DEM is still available as a separate branch in the repo. (Because it is nice to have fewer requirements.)