Atmosphere and Ocean state updates need to be more decoupled

[timspainNERSC]

Currently there is one overall function managing updates to the atmosphere and ocean states. In the case of a fixed flux implementation of IFluxCalculation the ocean state needs to be updated (at least once) to perform the salinity to freezing point calculation. The atmosphere, however, does not need to be updated and can be left entirely alone.

This will affect the slab ocean (issue #143).