NavMap filters: incremental, in place, and sparse layer updates

[fmrico]

Closes #123

• In place. The filters update the NavMap in NavState through get_ptr instead of copying it in and out.
• ObstacleFilter. It rebuilds its layer from the static map only when the map or its static layer changes. After that, it writes just the cells that become obstacles or stop being one.
• InflationFilter. It keeps an exact navmap::DistanceField, updated with the obstacle cells that appear or disappear, and rewrites only the cells whose cost changes. inflate_layer_u8 uses the exact field too.
• Maps manager publishing.
  • Each update carries only the changed cells, as a sparse NavMapLayer, or the whole layer if many changed.
  • Updates go out only when there are subscribers, and a new subscriber gets the whole layers.
  • The recorded changes are cleared at the end of each cycle.
• Fixes. NavMapFilter gets a virtual destructor, since plugins are deleted through it. The README now gives the obstacle filter's real plugin name, ObstaclesFilter.

Time per cycle, with the robot moving and a 720-beam laser that sees the map's walls and a person walking around:

Map	NavCels	ObstacleFilter	InflationFilter
home2, 5 cm	169 796	6.7 → 0.25 ms	13.8 → 0.66 ms
warehouse, 5 cm	297 012	11.8 → 0.24 ms	26.9 → 2.7 ms
parking garage	38 543	0.40 → 0.30 ms	1.20 → 0.01 ms

Two costs to know about:

• The first cycle of a map is slower, because the exact field paints a disc around every obstacle cell: 67 ms instead of 25 ms on home2.
• A scene where hundreds of cells next to dense static walls change every cycle is slower too: inflation takes 30 ms instead of 15 ms on home2.

New tests check that:

• a moving obstacle changes only its cells, and repeating a scan changes nothing;
• a new map rebuilds the layers;
• incremental inflation equals a full recomputation, and records exactly the cells that changed, over 40 random cycles;
• the maps manager publishes the whole layer to a new subscriber, nothing when nothing changes, and only the changed cells otherwise.

The simulation battery gives the same results in summit_warehouse_amcl, summit_bonxai_amcl and tiago_navmap_bonxai_amcl.