BlazePose 3D keypoints

[shiffman]

I'm just opening an issue here to link to and track the discussion at ml5js/ml5-website-v02-docsify#181. It started as a documentation discussion but has moved towards ideas for changing the model output for the z values of the BlazePose model. See latest proposal: ml5js/ml5-website-v02-docsify#181 (comment). I think we can do these as "non-breaking" changes. I am also going to work on a new example showing how to use the z data.

[ziyuan-linn]

I think we have three options:

1. Remove the z values for the named keypoints:

nose: {
  x: 332.6024622758805,
  y: 265.78330263473146,
  confidence: 0.9993924452777454,
}

2. Nest the 3d values in another object:

nose: {
  x: 332.6024622758805,
  y: 265.78330263473146,
  confidence: 0.9993924452777454,
  keypoint3D: {
    x: 0.05988978072436527,
    y: -0.5489126977664187,
    z: -0.26418375968933105,
    confidence:  0.9898926452717258
  }
}

3. prepend the 3d values with 3d:

nose: {
  x: 332.6024622758805,
  y: 265.78330263473146,
  confidence: 0.9993924452777454,
  3dX: 0.05988978072436527,
  3dY: -0.5489126977664187,
  3dZ: -0.26418375968933105,
  3dConfidence:  0.9898926452717258
}

I agree this can be a non-breaking change since removing the z can be viewed as a bug fix. Perhaps it should be a minor change instead of a patch?