Reduce allocations in the canopy update aux

[kmdeck]

Purpose

Reduces allocation in Canopy update_aux. Small improvement in time to solution (13%), but most of the solution time is now in updating the drivers, so this is potentially a larger improvement in RHS functions.

To-do

Content

1. Instead of computing esat, eair, and then relative humidity and allocating each, I wrote a function to compute the relative humidity, and we now broadcast that over existing fields when updating frac_diff in place.
2. allocate frac_diff in cache and update in place.
3. refactor update_photosynthesis
4. refactor compute_PAR, compute_NIR
5. refactor compute_absorbances (change structure of canopy cache)

---

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

[AlexisRenchon]

Nice, the code looks cleaner like that! Thanks!
Maybe add a line in NEWS.md?
I don't think there is any breaking change, right? (all internal)

[AlexisRenchon]

What labels would be good for this PR?
performance or code clarity?

[kmdeck]

What labels would be good for this PR? performance or code clarity?

I added a "performance" label for now :) and will add in the NEWS md before merging. Thanks!