R34 object detection map layer

node for creating a map layer to mark detected objects (using homer_object_detection) as obstacles

• update map_manager_node and MaskingManager to reflect changes in homer_mapnav_msgs/ModifyMap: for performance the message now can hold multiple polygons that are drawn on a map layer instead of just one. Also add a new subscriber to clear the masking layer without resetting the entire map.
• update detected_objects_map_layer_node to create a list of polygons (rectangles) that are created from segmented objects and are then passed to map_manager_node to draw on the map as obstacles