Improve handling of geometries with multiple polygons.

[NoaMillsUSDA-ARS]

There are two ways in which a user may successfully upload polygon geometries with the upload_geometry() function, even though those geometries cannot be used by gcdl to create polygon subsets because they contain multiple polygons.

First, when a user calls upload_geometry() on a file, rgeocdl does not perform any checks on the file contents to ensure that there is only one polygon before uploading the geometry and returning a single guid. If the file contains multiple geometries, then the user will receive an error when they call download_polygon_subset().

Second, when a user calls upload_geometry() on an sf or sfc object that contains multiple polygons, rgeocdl compares the area of the union of the polygons and the area of the convex hull. If the area of the polygons covers over 80% of the area of the convex hull, then rgeocdl uploads the union of the polygons. Otherwise, rgeocdl uploads each polygon individually and returns a guid for each polygon. This means that it is possible for a user to call upload_geometry() an sf or sfc object that contains polygons that are separate but close enough to pass the convex hull check. Then, upload_geometry() will return a single guid for a geometry with multiple polygons, and gcdl will return errors when the user uses that guid to call download_polygon_subset().