minor fixes

[rosepearson]

Fixing various issues spotted during run producing a set of 8m DEM tiles across Aotearoa New Zealand:

• [x] Print a warning not an error of now coarse DEM actually exists in the tile even if in the instruction file
• [x] Ensure clipping during the waterways stay to the DEM bounds on not just the catchment geometry
• [x] Update number of river points used in interpolation is less than the default 100. Print update.
• [x] Add optional compression value for the final output files if netCDF format. Default is still 1, but can be increased.