Spinup shows uncontrolled growth / loss in some cells

[mnlevy1981]

In the 1 degree companion run, there are some grid cells where biomass for some functional types drops uncontrollably:

in the 0.1 degree run, there is uncontrolled growth in some functional types while others go negative:

I think we need to introduce a floor for biomass, at which point we do not let it drop any further. It might be the case that preventing negative values will be sufficient to keep other groups from growing unbounded, otherwise will we need a cap?

For completeness, I'll also mention that we should investigate the forcing datasets at these points; it might be the case that the growth / loss is a natural response to the forcing we are providing.