Logistic model of root growth

[JunyanDing]

Description:

1. simulates the rooting depth of cohorts using a logistic model. See attached file for detail
   FATESHydroDynamicRoots.pdf
2. modified the corresponding routines so that the soil layers can have zero roots so that 1) that layer will be bypassed in 1D solver, 2) fine root water content in soil layer deeper than maximum rooting depth will be set to zero so, 3) handle zero (and infinite) situation in calculating rhizosphere shell conductance

issue #726

Fixes: #786

Collaborators:

Ryan Knox, Chonggang Xu, Charlie Koven

Expectation of Answer Changes:

Checklist:

• [ x] My change requires a change to the documentation.
• I have updated the in-code documentation .AND. (the technical note .OR. the wiki) accordingly.
• [x ] I have read the CONTRIBUTING document.
• FATES PASS/FAIL regression tests were run
• If answers were expected to change, evaluation was performed and provided

Test Results:

CTSM (or) E3SM (specify which) test hash-tag:

CTSM (or) E3SM (specify which) baseline hash-tag:

FATES baseline hash-tag:

Test Output:

[rgknox]

Site level comparison between this branch and master, similar settings, BCI.
rootdepth-v2_plots.pdf
Note, while these results are similar, I do not expect b4b results against base in regression testing. We have made small modifications to error caps.

[rgknox]

tests pass w/ expected fails: /glade/scratch/rgknox/tests_1010-193620ch