zero flux if zero lai

[kmdeck]

Purpose

When LAI = SAI = RAI is zero, we still have a nonzero net LW radiation for the canopy, a SHF for the canopy, and nonzero autotrophic respiration.

To-do

[X] Go over with Alexis and Renato [X] Update tests

Content

1. In the canopy parameters, we define canopy emissivity. We now scale that by a factor following CLM as emissiv = emissiv_0*(1-exp(-(LAI+SAI)). This behaves correctly: as LAI+SAI -> 0, emissivity -> 0 and LW absorbed by the canopy -> 0. @braghiere
2. Autotrophic respiration now uses the stem area index to compute Ns, which is zero if SAI is zero (check with @AlexisRenchon)
3. Autotrophic respiration was before combining GPP with Rd. But, GPP is computed as An scaled by LAI, while Rd is at level of An. I am guessing GPP is canopy level, while An/Rd are leaf level. A possible fix (implemented) is to use An in computing the respiration instead of GPP, and then scaled to canopy at the end. Then we would satisfy that respiration is zero as LAI -> 0. Im not sure if this should be LAI+SAI, though. @AlexisRenchon
4. SHF change would would be easier to discuss in person @braghiere
5. we no longer use root area index, also easier to discuss in person @braghiere.

Review checklist

I have:

• followed the codebase contribution guide: https://clima.github.io/ClimateMachine.jl/latest/Contributing/
• followed the style guide: https://clima.github.io/ClimateMachine.jl/latest/DevDocs/CodeStyle/
• followed the documentation policy: https://github.com/CliMA/policies/wiki/Documentation-Policy
• checked that this PR does not duplicate an open PR.

In the Content, I have included

• relevant unit tests, and integration tests,
• appropriate docstrings on all functions, structs, and modules, and included relevant documentation.

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• [X] I have read and checked the items on the review checklist.

[braghiere]

All looks good.