Streamline Slab types

[kmdeck]

Currently, slab models are used for land (in some cases), sea ice, and ocean. Land can also be a bucket model, and ocean temperatures can also be prescribed. In the future, there will be different sea ice models as well.

• use multiple dispatch for obtaining T, energy, etc correctly from different models, as they are stored differently in different models
• make a more useful slab_utils file and separate ocean, land, and ice directories?

Basically, clean up duplicate code between slab models and make use of dispatch over flags for different model types.

[LenkaNovak]

To be addressed as part of #212