 <h3>ML columns</h3>
 *
 * A column with ML attributes attached is called an ML column.
 * The data in ML columns are stored as double values, i.e., an ML column is either a scalar column
 * of double values or a vector column.
 * Columns of other types must be encoded into ML columns using transformers.
 * We use {@link org.apache.spark.ml.attribute.Attribute} to describe a scalar ML column, and
 * {@link org.apache.spark.ml.attribute.AttributeGroup} to describe a vector ML column.
 * ML attributes are stored in the metadata field of the column schema.
 <h3>ML columns</h3>
 *
 * A column with ML attributes attached is called an ML column.
 * The data in ML columns are stored as double values, i.e., an ML column is either a scalar column
 * of double values or a vector column.
 * Columns of other types must be encoded into ML columns using transformers.
 * We use {@link org.apache.spark.ml.attribute.Attribute} to describe a scalar ML column, and
 * {@link org.apache.spark.ml.attribute.AttributeGroup} to describe a vector ML column.
 * ML attributes are stored in the metadata field of the column schema. | package-info.java