Make route_grid_publisher node

[jruths]

• Currently the grid_summation_node subscribes to the /planning/route topic and constructs a grid that highlights the route as low-cost. This should get pulled out of the grid_summation_node and into a new node that publishes /grid/route_distance (which will feed into the grid_summation_node).
• The current implementation of this just assigns a narrow low cost area, but it would be good to roughly fade this cost as it goes away from the route.