get length(): number {
    return Math.max(0, ...this.columns.map(k => this.get(k).length).toArray());
  }
Return the length of the `DataFrame`

pandas equivalent: len(df);

@returns {number}

@example
const df = new DataFrame([{x: 1, y: 2}, {x: 2, y: 3}, {x: 3, y: 4}]);

// Returns 3
df.length;