code: function (v) {
              var dx = this.x - v.x, dy = this.y - v.y, dz = this.z - (v.z || 0);
              return Math.sqrt(dx * dx + dy * dy + dz * dz);
          }
docs: return the distance between this vector and the passed one
  @name distance
  @memberOf me.Vector3d
  @function
  @param {me.Vector2d|me.Vector3d} v
  @return {Number}