Add octree I/O support for json

[yxlao]

Add octree I/O support for json (ascii output). Based on #903, merge #903 first.

Example json output

{
    "class_name" : "Octree",
    "max_depth" : 1,
    "nodes" : 
    [
        {
            "children" : [ 1, 2, 3, 4, 5, 6, 7, 8 ],
            "class_name" : "OctreeInternalNode",
            "id" : 0
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.0, 0.0, 0.0 ],
            "id" : 1
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.25, 0.0, 0.0 ],
            "id" : 2
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.0, 0.25, 0.0 ],
            "id" : 3
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.25, 0.25, 0.0 ],
            "id" : 4
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.0, 0.0, 0.25 ],
            "id" : 5
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.25, 0.0, 0.25 ],
            "id" : 6
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.0, 0.25, 0.25 ],
            "id" : 7
        },
        {
            "class_name" : "OctreeLeafNode",
            "color" : [ 0.25, 0.25, 0.25 ],
            "id" : 8
        }
    ],
    "origin" : [ 0.0, 0.0, 0.0 ],
    "root_node" : 0,
    "size" : 2.0
}

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This change is 

[qianyizh]

Reviewed 7 of 7 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @germanros1987, @syncle, @takanokage, and @yxlao)

a discussion (no related file):
Can we make it a recursive json structure?
Then you just need to make OcTreeNode as IJsonConvertible, then you call Traverse, it solves the problem, yes?

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[syncle]

Reviewable status: 4 of 6 files reviewed, 1 unresolved discussion (waiting on @germanros1987, @qianyizh, and @takanokage)

a discussion (no related file):

Previously, qianyizh (Qianyi Zhou) wrote…

Can we make it a recursive json structure?
Then you just need to make OcTreeNode as IJsonConvertible, then you call Traverse, it solves the problem, yes?

Same comment. Other than that, it looks good to me.

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[qianyizh]

@yxlao any follow up on this?

[yxlao]

@qianyizh comments addressed

[qianyizh]

Reviewed 5 of 5 files at r2.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @germanros1987, @qianyizh, and @takanokage)