addGeomData: Put geometry data in a NetCDF-CF File
In dblodgett-usgs/NCDFSG: NetCDF-CF Simple Geometries

Creates a file with point, line or polygon instance data ready for the extended NetCDF-CF timeSeries featuretype format. Will also add attributes if a sp dataframe object is passed in.

1	addGeomData(nc_file, geomData, instance_dim_name, variables = c())

`nc_file`	A string file path to the nc file to be created. It must already have an instance dimension.
`geomData`	An object of class `SpatialLines` or `SpatialPolygons` with WGS84 lon in the x coordinate and lat in the y coordinate. Note that three dimensional geometries is not supported.
`instance_dim_name`	A string to name the instance dimension. Defaults to "instance"
`variables`	A character vector of variable names that the geometry data container variable name will be added to.