Make nc3crop and nc3irrig private to pftconMod

[samsrabin]

Description of changes

As part of our goal to make our code agnostic to the ordering of PFTs (#21) and avoid hard-coded PFT properties (#3388), this PR makes the nc3crop and nc3irrig variables (PFT indices for the generic crops) private to pftconMod.

Specific notes

Contributors other than yourself, if any: None

CTSM issues resolved or otherwise addressed, if any:

• Contributes to refactor code that assumes a particular ordering of PFT type constants #21
• Contributes to Avoid in-code checks of PFT type #3388

Any user interface changes (namelist or namelist defaults changes)? No

Testing planned or performed, if any:

• aux_clm
• fates (just to make sure some new variables I added aren't ever needed under FATES)
• SMS_Ln9_P256x3.f19_g17.IHistClm50Sp.derecho_intel.clm-waccmx_offline2005Start, SMS_Lm1.f19_g17.I1850Clm60BgcCropCmip6waccm.derecho_intel.clm-basic, SMS_Lm1.f10_f10_mg37.I1850Clm60BgcCropCmip6waccm.derecho_gnu.clm-basic

Requirements before merge:

• I have followed the CTSM contribution guidelines.

• The code in this PR branch builds with no errors.
• The code in this PR branch runs with no errors. Briefly describe tested configuration(s):
• This either (a) does not change answers, (b) it only changes answers at roundoff level, or (c) I have performed a scientific evaluation of the answer changes. Which?: Doesn't change answers

• I have reviewed relevant parts of the CLM documentation Tech Note or User's Guide to determine if anything needs to be changed or added. If it does, describe:
• This PR either (a) does not create a need to update the documentation or (b) includes required documentation updates (see guidelines for contributing documentation). Which?: No need

[wwieder]

@adrifoster asked if adding an extral moss PFT is critical for the ABOVE project, as there are unused PFTs for the Arctic sites she's running (which include "tropical trees") and could be adopted for moss.

Long-term this is something we'll need, but it's a larger project.

@samsrabin would like to bring this PR in, as it's mostly done, which seems appropriate.

[slevis-lmwg]

Approving preemptively but asking for a guided tour of a few items. Thanks @samsrabin :-)