demo plugin for koha plugin hook for framework_defaults_override
this plugin has the following options $tag, $subfield, $value, $form_mode
ie marc tag, marc subfield, the "before value", the action which triggered the change
possible actions are new, duplicate, changed (Framework), imported (via SRU)
the plugin has to return a hashref containing two keys
apply_override - undef or a true if the default value from the framework should be applied override_default_value - a completly new derived value