Backport weight-per-block set by file from CICE6

[nichannah]

Presently the amount of work done in each block is estimated as a linear function of latitude. This is obviously not a very close approximation to the amount of ice work at a given geographic location.

CICE6 has an option to set the work weight for each grid point using a file containing a 2d field. We think this will allow a lot more accurate specification of the amount of work and hence better load balancing.

This issue will back-port the CICE6 functionality to our version of CICE.