Description of changes

See issue #608 for discussion of the original user-reported problem that generated this investigation and solution.
Allow for reseeding of generic crops and increase the totvegc threshold that triggers this from 0 to 1 gC/m2.
This is a work-around solution that addresses the issue of dead generic crops in an accelerated-mode BGC simulation initialized from our current BGC-Crop initial files.

A remaining problem not addressed by this request is that there are generic crops present in our surface datasets that are used by BGC-CROP simulations. This is not desirable as all crops should be distributed among the prognostic crop types. This issue will be addressed by either continued discussion in issue #608 or by opening another issue.

Contributors other than yourself, if any: D. Lawrence, P. Lawrence, E. Kluzek

CTSM Issues Fixed (include github issue #): work-around fix for #608

Are answers expected to change (and if so in what way)? Yes, for BGC generic-crop cases that are initialized from our current normal mode BGC-CROP initial files that are then taken into accelerated mode.

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

Testing performed, if any:
I reproduced the user's case referenced in issue #608 and verified the solution:
./create_newcase --compset I2000Clm50BgcCrop --res hcru_hcru
No other testing (due to the expense of the full test suite and my assumption that this would likely be folded into a tag with other changes.)