code: function interpolate (dyear, data) {
    const d3 = interp.len3ForInterpolateX(dyear,
      data.first, data.last, data.table
    )
    return d3.interpolateX(dyear)
  }
docs: interpolation of dataset
  @private
  @param {Number} dyear - julian year
  @returns {Number} ΔT in seconds.