[points] Add operator > for points_t (use size)
[nav_graph] Return a valid cost for the path
[exploration] Largely rewrite the API

- Computing frontier takes more parameters.
- Frontiers may be filtered.
- New attributes (warning : this slows the computation).
- No more sorting here (it is not the purpose of this module !)
- Use a nav_graph (and its weight map) : this was required for some attributes
computation.

Update python bindings and unit test accordingly.