Concatenates results of multiple transformer objects.
This estimator applies a list of transformer objects in parallel to the
input data, then concatenates the results. This is useful to combine
several feature extraction mechanisms into a single transformer.
Parameters of the transformers may be set using its name and the parameter
name separated by a '__'. A transformer may be replaced entirely by
setting the parameter with its name to another transformer, removed by
setting to 'drop' or disabled by setting to 'passthrough' (features are
passed without transformation).