addFeature(feature) {
    this.addFeatureInternal(feature);
    this.changed();
  }
Add a single feature to the source.  If you want to add a batch of features
at once, call {@link module:ol/source/Vector~VectorSource#addFeatures #addFeatures()}
instead. A feature will not be added to the source if feature with
the same id is already there. The reason for this behavior is to avoid
feature duplication when using bbox or tile loading strategies.
Note: this also applies if a {@link module:ol/Collection~Collection} is used for features,
meaning that if a feature with a duplicate id is added in the collection, it will
be removed from it right away.
@param {FeatureType} feature Feature to add.
@api