Update the Section default options for land-width parsing and cube geometries

[amoodie]

I know this is a 'temporary' method until we add the land width to the netCDF file, but could this be called during cube initialization and held in the cube as self._trim_idx or something similar? Might make the swap down the line easier to implement as the function would be called once during initialization instead of spread across multiple files/methods?

(#26 (comment))

Probably a good idea, and related to a bug that sprung up on the RadialSection too.

After updating the netcdf file, create some attributes of the cube based on whether those attributes exist in the netcdf file will make the most sense.

For example, if L0 is in the netcdf, then make self.L0 in the Cube and then when creating a Section, you can check whether Cube.L0 exists, and use it if it does.