Make npcropmin/max private to pftconMod

[samsrabin]

Description of changes

Making npcropmin and npcropmax private to pftconMod helps set up for adding an is_prognostic_crop parameter to the parameter file, contributing to #3388. Making these private required adding an is_prognostic_crop() function, the contents of which will eventually be replaced by a read of that new parameter.

Specific notes

Contributors other than yourself, if any: None

CTSM Issues Fixed:

• Contributes to Avoid in-code checks of PFT type #3388

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so? No

Testing performed, if any:

• aux_clm passes

[slevis-lmwg]

I made two minor suggestions, and I am approving preemptively. Thank you @samsrabin for this work!