Build mkgmap/splitter #187

improve split: - don't split single tile which already has less than --max-nodes nodes - improve handling of bounding boxes which span planet (-180 .. 180) when no polygon is given: don't create small gaps between tiles. Value in minEmpty is now 10° instead of 10 grid elements - reset threshold values when switching between the two alternative search algortihmn This allows to find good splits for problematic downloads from geofabrik like asia or australia-oceania.