dynamic root code divides by 0

[bishtgautam]

Reported by CSEG at http://bugs.cgd.ucar.edu/show_bug.cgi?id=2237:

Runs that activate both use_dynroot and crop crash in the first time step if run in debug mode. This is >due to a divide by 0 error in this line in CNRootDynMod:

               root_depth(p) = max(zi(c,2), min(hui(p)/huigrain(p)* root_dmx(ivt(p)), root_dmx(ivt(p))))

huigrain(p) can sometimes be 0; in the one case I looked at, hui was also 0. Also, in the first timestep of a cold start, huigrain is NaN.

So this needs:

(1) initialization of huigrain in cold start so that it isn't NaN: it should probably be initialized to 0

(2) appropriate handling of the case when huigrain = 0

[bbye]

@bishtgautam and @xyk-ornl I fixed this error, but when I went to test it I found some more errors that were introduced by the phosphorus code, which caused the crop model to fail. Do you want me to import all the bug fixes in one PR or break it into two?

[bishtgautam]

@bbye : You can create a new issue (for e.g. Crop model incompatible with Phosphorus cycle). Then, issue a single PR that fixes this and the new issue. If possible, have two separate commits in the PR that fix each issue independently.

Later, if you send a set of commands to create a case with crop model active, I will try to add a new test to ensure current capabilities of crop model aren't broken as new PRs are merged to create the ACME v1.