Possible inaccurate calculation of diffusivity output

### Brief summary of bug

As [mentioned](https://github.com/ESCOMP/CTSM/issues/1990#issuecomment-1664606259) in #1990, the nitrification-denitrification code saves a version of diffusivity that is only used in the output field `diffus`. It uses a function dependent on soil temperature for this, which requires two coefficients. The coefficients used are for oxygen, but it may be that it would be more appropriate to use coefficients for N(2)O instead.

### General bug information

**CTSM version you are using:** `ctsm5.1.dev130-6-gc126e2e27`

**Does this bug cause significantly incorrect results in the model's science?** Unsure

**Configurations affected:** All?

### Details of bug

Figuring this out may require a fair amount of scientific effort. I spent a few hours looking through literature the other day and didn't find any ready-to-use parameters for the equation in question, although this isn't my area of expertise.

The coefficients in question are the `d_con_g` members in the first line here, from `SoilBiogeochemNitrifDenitrif()`:
https://github.com/ESCOMP/CTSM/blob/f47ce5cf1ab0223670e13f2ebccd876ec98ec09d/src/soilbiogeochem/SoilBiogeochemNitrifDenitrifMod.F90#L281-L283

Note that, as of ctsm5.1.dev132, this version of `diffus` (i.e., units m2/s instead of unitless) is incorrectly used later in `SoilBiogeochemNitrifDenitrif()`. As described in #1990, I will soon submit a PR to fix that.