Add docs and PEP8 clean-up to `seaice` modules

[xylar]

This PR adds the seaice modules to the documentation, adds docstrings to each "public" function, makes some functions explicitly "private" by adding a _ in front, and reformats the code using autopep8 and some hand editing to conform to the conventions of other modules in the package.

[xylar]

@akturner, thanks for the suggested edits. I have made them all. I have also rebased in case you want to have a look here.

I will merge once CI is done.