[frontier] Fix "jump" in frontiers computation.

Appeared when scale is not equal to one.
[frontier] Use internal parameters to simplify function calls.
[frontier] Add a maximal size for the frontiers.

Avoid too big frontiers (split them).
(typically : one big circle around the robot).

May "minimize" the impact of some issues with the lookout choice.
(And delegate the decision to higher levels, like xares decision making
function, which is a good thing).