"Quantizer.quantize" should return an instance of "ColorMap".
Move "Octree" and "OctreeQuantizer" to "jp.mitsuse.electron.octree".
"Octree" requires its max depth.
Remove implementations of "Octree.add" and "Octree.get".
"Octree.add" and "Octree.get" require color as their argument.
"OctreeQuantizer" requires the max depth of octree.
"Octree.add" stores the color and its frequency.