TruncateCandNStates should abort if just one of C or N is near-zero

**Bill Sacks < sacks@ucar.edu > - 2017-01-26 14:22:25 -0700**
**Bugzilla Id:** [2416](http://bugz.cgd.ucar.edu/show_bug.cgi?id=2416)
**Bugzilla CC:** andre@ucar.edu, dlawren@ucar.edu, oleson@ucar.edu, rfisher@ucar.edu,

In CNPrecisionControlMod: TruncateCandNStates: Currently, if either C or N is near-zero, then both are set to 0. This can cause errors to be swept under the rug, when C & N are in inconsistent states.

We agreed in yesterday's clm-cmt meeting that the behavior should be changed so that: If C is within roundoff of zero but N is substantially bigger than 0, then we should abort with an error message - and vice versa.

The main choice to make is how to define "substantially bigger than 0".

I said I'd take this on, but my plate is overflowing, so I'm filing this as something to come back to.