Extend geofence_transform to accept a directory name containing .gml files and use the combined polygon as the EEZ boundry

[webbpinner]

Add a boundary_dir_name argument for specifying a directory containing *.gml files that will be used for building a union-ed polygon for EEZ detection. The transform should accept either a boundary_file_name OR boundary_dir_name... not both.

This functionality will allow users to download a library of gml files and control which files are applied by copying OR symlinking the files to a directory that the geofence transform reads on startup.