Improve diagnostics

[mnlevy1981]

This PR addresses two small issues with diagnostics:

1. fixes #401 (photoC_NO3_TOT_zint_100m was being defined but never computed)
2. resolves #400 (pocToFloor is now output in the medium stream by default)

For the first, I opted to compute photoC_NO3_TOT_zint_100m rather than remove it from the diagnostic list.

There are two small issues remaining to address:

• I still need to update the baseline file to avoid an error when comparing results from tests/regression_tests/call_compute_subroutines/
• There is a CI test failure in building the documentation that I think stems from an incompatibility between the version of sphinx we are using for the docs and a major version update of jinja2.

I'll remove draft status from this PR after fixing those two minor issues.

[mnlevy1981]

After talking with @klindsay28, we thought the best plan would be to

1. Bring in the CI changes in a separate PR (#404)
2. Discuss the two other changes in this PR on Tuesday

I'm going to close this PR, and will open a new one (or two) next week, depending on how the conversation goes.