finds the first row with the most columns
function findHeader(aoa) {
  const {i} = aoa.reduce(
    (prev, row, i) => {
      const len = rowLength(row)
      if (prev.len < len) {
        return {i, len}
      }
      return prev
    },
    {i: -1, len: 0}
  )
  return i
}