PR #3409. Description:
Intoduce code, namelist option, and tests on the CLM side to accommodate interactive nitrogen with FATES.

Roundoff changes for most cases, though not for Sp and FatesSp.

The FATES side PR is https://github.com/NGEET/fates/pull/1472. Fates got updated from sci.1.92.1_api.44.1.0 to sci.1.92.4_api.45.0.0.

Supporting information (other than in the issue referenced below):
- Nutrient enabled FATES handbook: https://docs.google.com/document/d/1I35fGDfKTkn9_8Z6qXot7HZf3ICSpLONd-iFEErAh0k/edit?usp=drive_link
- FATES CLM N coupling: https://docs.google.com/document/d/1mpBtpCLGJpAGw6R3-nGGY92IWRU3ISvaAUKxvcDVJRw/edit?usp=drive_link

Resolves issue #3378.