Explore hidden areas at first

[DmytroKushnirSoftserveinc]

From the active_SLAM paper :
idea is to have a separate class of frontiers which are within the lidar range, but is not mapped: the usual reason is occlusion from some obstacles. As those areas are generally cheapest to explore it makes sense to search them at first.
-- done

-- upd Iter 1 completed

[DmytroKushnirSoftserveinc]

Idea: use a shorter timeout for marking them as unreachable, as they are potentially situated at the narrow paths.
-- done

[DmytroKushnirSoftserveinc]

Idea: try to reach the 'oldest' "Hidden" areas at first in order to minimize the cases when you have to plan your way back to the unmapped piece of territory left behind.

[DmytroKushnirSoftserveinc]

Solve the problem when we have a wall on a way to close frontier (when frontier is in the other room).
OR: do not mark such frontiers as hidden at all

[DmytroKushnirSoftserveinc]

the idea of hidden frontiers need more exploration, implemented version seems like the only helper for heuristic ranking... as opened but still underexplored area is evaluated as such;
Contrary: it gives 'chained' exploration effect, but better to do it with nice heuristics.
UPD: shorter distance seems to be a better solution, but still suboptimal.