code: function interpolateData (dyear, data) {
    const [fyear, fmonth] = data.firstYM
    const {year, month, first, last} = monthOfYear(dyear)
    const pos = 12 * (year - fyear) + (month - fmonth)
    const table = data.table.slice(pos, pos + 3)
    const d3 = new interp.Len3(first, last, table)
    return d3.interpolateX(dyear)
  }
docs: interpolation of dataset from finals2000A with is one entry per month
  linear interpolation over whole dataset is inaccurate as points per month
  are not equidistant. Therefore points are approximated using 2nd diff. interpolation
  from current month using the following two points
  
  @private
  @param {Number} dyear - julian year
  @returns {Number} ΔT in seconds.