Can not arbitrarily set CICE cores

[manodeep]

While the (my) intent of allocated_unused_cores_to_CICE boolean was to improve resource usage, there are constraints on the number of cores that CICE can use - specifically, the number cores has to divide the nxglob (=360). That constraint translates to only being able to use one of these numbers `1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120, 180 and 360 - we would presumably pick the largest factor that still fits within the available cores for CICE.