Snow grain radius out of bounds

[njeffery]

If the snow grain radius is set to zero, possibly because of zapping small ice or if ice disappears mid-timestep, then updates of snow grain radius will become NaNs and the code will crash.

For most places in the code, snow grain radius is bounded between a min and max so this generally doesn't happen. A recent coupled bgc run did crash though with this error.

To correct this, I'll need to modify Icepack.