NavMap A* ignores the terrain: slopes, steps, low ceilings and links between surfaces #129

The NavMap planner only weighs the length of a path and the cost of the inflated layer. It treats a steep slope and flat ground as equal, as long as both are in the map, and it cannot take links between surfaces such as stairs or elevators. NavMap now provides per-NavCel descriptors (slope, step, roughness, clearance) and links (EasyNavigation/NavMap#35), but the planner does not use them.