262 performance improvement separate out raster generation for each waterway

[rosepearson]

• Split out waterways DEMs one for each waterway

DESCRIPTION OF PR:

• [x] Make code change
• [x] Update tests
  • [x] Update / create new tests
  • [x] Ensure these tests have the expected behavour
  • [x] Test locally and ensure tests are passing
• [x] Ensure tests passing on GH Actions
• [x] Update documentation
  • [x] Doc strings
  • [x] Wiki
• [x] Update package (version.py & pyproject.toml) version (if needed)