Stability issue in dynamicLagrangian model

Hi,

There is a stability issue in the dynamicLagrangian model, which is caused by flm_/fmm_. Some cells may have 0 value in fmm_. If replacing flm_/fmm_ with flm_/(fmm_+fmm0_), the model will be stable, where fmm0_ is a VSMALL defined in the model.

Attached is a patch file for this model.V1806-Fix-stability-issue-in-dynamicLagrangian-model.patch

Thanks,

Ning Ren

## Reattaching the author to the issue ticket: @renning ##