INotify Interface

Notification triggering in `PluginController` can in principle work without registration. This is a rewrite to allow a bit more flexibility (for multi plugins without check pointing of their state).

(I know this would not work for a `INotify` object that needs checkpoint/restart triggering - this would  need a full flavoured `IPlugin`.)

to do:
- [x] run time testing (lwfa, including multiple png `slicePoint`s per slice)