Fixes perfect restart issue in the ICE_SHELF module

When the ice shelf thickness changes in time (e.g., DYNAMIC_SHELF_MASS = True), fluxes applied under the ice shelf during the previous time step should be set to zero. Otherwise, points that no longer have a floating ice shelf will carry these fluxes, which is a problem (e.g., it won't allow perfect restarts). This commit adds the following:

** Zero out fluxes applied under an ice shelf in the previou time step.

** Adds additional checksums for debugging.