public FieldValue getFieldValue(String fieldName) {
        Field field = getField(fieldName);
        if (field == null) return null;
        return getFieldValue(field);
    }
Convenience method to return the value of a named field. This is the same as calling {@link #getField(String)},
and using the returned value to call {@link #getFieldValue(Field)}. If the named field does not exist, this
method returns null.

@param fieldName the name of the field whose value to return.
@return the value of the field, or null if it is not declared in this, or has no value set
document/src/main/java/com/yahoo/document/datatypes/StructuredFieldValue.java