Gridgeo

* CFVariable axis methods will error out when more than 1 coords is found.

* Fixed non-string properties bug in 40

* Implemented most of netCDF4.Variable methods and properties in CFVariable.
Things like var.shape or var.size will work by default without the need to
request the original variable object.

* Return the array of polygons and the shapely geometry.
* Parse the CRS using the CF grid_mapping attribute.
* Use the newer and faster unary_union instead of cascaded_union to compute the grid outline.

* Cast connectivity array to int.

* 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'.