[doc] add gdalwarp scale to 50cm
[frontier] Searching for the Lost Segfault...
[frontier] various updates

- weight_map: add index_utm
- negative coordinates
- unknown points, etc.
[frontier] update API

Take new parameters into account to deal with frontier computation.
Enable frontier filtering.
Update frontier attributes and their computation.
Correctly handle assertions.
[frontier] enhance display
[frontier] Add lookout to the path
[frontier] Add yaw consideration

- Explicit the yaw computation
[frontier] Bounded area to consider for frontiers detection
[frontiers] Fix how to test against unknown cells
[test] test_frontier is now valid again