[weightmap] Add an index function.

index = x + y * width (useful and readable)
[exploration] Add a new frontier detector.

Use for exploration planning using frontiers.
Also compute the frontiers attributes.
[test] Exploration unit test.
[python] Exploration Python bindings and API.

Thanks to Pierrick Koch for his review and help.
Implement simple LRU cache system
[visibility] Add EPS(ILON) for float comparisons
[visibility] Take into account known/unknown state of the points
[test] update visibility unit test

Take into accourt the unknown/known state for visibility.