Allow for ability of line and area charts to interpolate between points

[SamGRosen]

Currently when drawing lines and area charts, the interpolation between points is a line, as that is the default behavior when drawing lines with WebGL. Interpolating between the points non-linearly has value. This could be accomplished potentially by adding a check to the line case in VertexCalculator._calculateForMark. Knowledge of the previous mark would be necessary for interpolation but this is already done when calculating area charts. The case for lines would likely look something like this:

if (this.track.mark === "line") {
  let toReturn;
  if(this.track.interpolation === "linear" || !this.track.interpolation) { // default
    toReturn = this._getVertexForDot(mark);
  } else if(this.track.interpolation === "basis") {
    toReturn = this._getVerticesForInterpolatedLine(mark, "basis")
  }
 this.lastMark = mark;
 return toReturn;
}
...
_getVerticesForInterpolatedLine(mark, interpolationType) {
  const interpolateFunc = getInterpolationFunctionBetweenPoints(
     this._mapToGPUSpace([mark.x, mark.y]), 
     this._mapToGPUSpace([this.lastMark.x, this.lastMark.y]), 
     interpolationType
  ); // takes t in [0, 1] where t = 0 -> mark.xy and t -> 1 this.lastMark.xy
  let vertices = []
  for(let i = 0; i < NUM_POINTS_FOR_INTERPOLATION; i++) {
    vertices.push(...interpolateFunc(i / NUM_POINTS_FOR_INTERPOLATION));
  }
  return vertices;
}

Be sure to add an example using the interpolation and record the expected output for future integration tests.