fit(dataset) {
    const t0 = performance.now()

    const n = dataset.length
    console.log('Fitting to ', n, 'rows')
    for(var i = 0; i < n; i++) {
      const label = dataset[i][this.column]
      if (! this.label_indices.hasOwnProperty(label)) {
        this.label_indices[label] = this.label_counter
        this.index_labels[this.label_counter] = label
        this.label_counter++
      }
    }

    const t1 = performance.now()
    console.log("LabelEncoder.fit took " + (t1 - t0) + " milliseconds.")
  }
Fit to the data.
@param   {Array[Object]}  dataset  Array of Objects, each of the objects must
                                   contain this.column property
@return  {void}