Topic based timeout of obstacles

Updated the obstacle timeout so that you can specify to clear old obstacles based on the topic 

Enable this by setting the following for each sensor for which you require the obstacles to timeout (this only works for global maps) 
e.g.
bump: { clear_old: true, max_obstacle_persistance: 3.0}

Time is in seconds.

Currently - it will clear the map if an obstacle in either layer times out - i.e. height of different sensors is not respected.