include snow drainage in model output

[martynpclark]

Make sure all the relevant boxes are checked (and only check the box if you actually completed the step):

• [ ] Closes #xxx (identify the issue associated with this PR)
• [ ] Code passes standard test cases (results are either bit-for-bit identical, or differences are explained in the PR comment)
• [ ] New tests added (describe which tests were performed to test the changes)
• [ ] Science test figures (add figures to PR comment and describe the tests)
• [ ] Checked that the new code conforms to the SUMMA coding conventions
• [ ] Describe the change in the release notes (use either ./summa/docs/whats-new.md or ./summa/docs/minor-changes.md depending on what changed)