Child classes should be used to override this function and set the 'value options', unless 'options callback' is defined as a valid function or static public method to generate these values.
This can use a guard to be used to reduce database hits as much as possible.
Return the stored values in $this->value_options if someone expects it.
Overrides InOperator::get_value_options
function get_value_options() {
/* don't overwrite the value options */
}