unique(): Immutable.List {
    return this.values.toSet().toList();
  }
Returns `Immutable.List` of unique values in the `Series`. Preserves order of the original

pandas equivalent: [Series.unique](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.unique.html)

@returns {List}

@example
const ds = new Series(['foo', 'bar', 'bar', 'foo', 'foo', 'test', 'bar', 'hi']);
// Returns ['foo', 'bar', 'test', 'hi']
ds.unique();