public VectorExpressionWriterSetter init(
        SettableStructObjectInspector objInspector,
        StructField fieldRef,
        VectorExpressionWriter baseWriter) {
      this.fieldRef = fieldRef;
      this.settableObjInspector = objInspector;
      this.objectInspector = fieldRef.getFieldObjectInspector();
      this.baseWriter = baseWriter;
      return this;
    }
VectorExpressionWriterSetter helper for vector expression writers that use
settable ObjectInspector fields to assign the values.
This is used by the OrcStruct serialization (eg. CREATE TABLE ... AS ...)
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/VectorExpressionWriterFactory.java