code: function GLTFCubicSplineInterpolant( parameterPositions, sampleValues, sampleSize, resultBuffer ) {
  
  		THREE.Interpolant.call( this, parameterPositions, sampleValues, sampleSize, resultBuffer );
  
  	}
docs: Spline Interpolation Specification: https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#appendix-c-spline-interpolation