Refactor SlabSeaIceModel to provide an Oceananigans-style interface into equation configuration

[glwagner]

SlabSeaIceModel evolves one essential prognostic variable, the ice thickness h.

It should also evolve another sometimes essential variable, the vertically-averaged salinity S.

Finally, it should also support tracers.

It'd be nice to provide an Oceananigans-style interface for controlling the behavior of the thickness equation, the salinity equation, and the tracer equations, including optional user-defined forcing.

This implementation probably also needs to come with documentation that gives a mathematical description of the SlabSeaIceModel.