Add an option to shrink the borders of all affordance of a given size

Created by: pFernbach

See https://github.com/humanoid-path-planner/hpp-affordance/pull/3 Add an optionnal argument in LoadObstacleModel : reduceSizes This argument is a vector that define the desired size for the reduction of the Affordances plane. The first element is for the 'Support' affordance and the second for the 'Lean' affordance.

We should improve the API if we add more affordance types.