[global_planner] tolerance implementation

[doisyg]

Implementation of the tolerance param which was never used. See https://github.com/ros-planning/navigation/issues/262, https://github.com/ros-planning/navigation/pull/703 and https://github.com/ros-planning/navigation/pull/1040

First naive and not so optimized implementation which checks validity using calculatePotentials after filtering out lethal cells (so potentially CPU heavy for large tolerance). Alternatively, a check for the cells value could be used without using calculatePotentials, or also filtering inscribed or even circumscribed cells. I am up for the debate.

[derektan95]

Hi, could we please have these changes in ROS Melodic as well? It will be really helpful, thanks.