Inflation layer option to inflate unknown space

[SteveMacenski]

This can be helpful when you don't want to traverse into unknown to inflate those costs to steer you out of them in case there's any issue in being near them.

Also useful for working with maps without occupancy information. It can be implmeneted by changing:

      if (cost == LETHAL_OBSTACLE) {
        obs_bin.push_back(CellData(index, i, j, i, j));
      }

to

      if (cost == LETHAL_OBSTACLE || (cost == UNKNOWN && inflate_unknown_)) {
        obs_bin.push_back(CellData(index, i, j, i, j));
      }