Generate and send non-overlapping modifications.
Generate and send non-overlapping modifications.

The TR-55 code needs a set of modifications with the following property:
"No point on the map is covered by more than one 'BMP' polygon, nor by
more than one 'reclassification' polygon."  This patch introduces a
function to compute such a set, applies it to the list of modifications,
and sends the output to the back-end TR-55 code.

Connects #608

lliss/model-my-watershed,WikiWatershed/model-my-watershed,kdeloach/model-my-watershed,kdeloach/model-my-watershed,WikiWatershed/model-my-watershed,kdeloach/model-my-watershed,WikiWatershed/model-my-watershed,lliss/model-my-watershed,WikiWatershed/model-my-watershed,kdeloach/model-my-watershed,lliss/model-my-watershed,kdeloach/model-my-watershed,WikiWatershed/model-my-watershed,project-icp/bee-pollinator-app,project-icp/bee-pollinator-app,project-icp/bee-pollinator-app,project-icp/bee-pollinator-app,lliss/model-my-watershed,lliss/model-my-watershed