* Construct the grid based on the variables rather the datasets.
The variables are CF's smallest unit and the only way to get a unique valid grid for SGRID.
* Dropped the dependency on iris, cartopy, pysgrid, and pyugrid.
* Dropped raster object in lieu of the actual x,y or triangle data for plotting.
* Added an option to reduce the precision for the GeoJSON.
* Grid parsing is now done with a grid-aware CFVariable object.
* Dropped parsing grid from iris.cube.
* Grids names are 'ugrid', 'sgrid', 'unknown_1d', and 'unknown_2d'.