Provide an interface for "incremental" models

[arcanefoam]

When using incremental execution in "on-line" mode, the models needs to support a notification mechanism of some sort. The requested interface should provide methods for enabling the notification mechanism and registering listeners. Listeners should implement the interface proposed in #11 .
For "off-line" mode, the interface should provide rapid access to model properties, this can be used for example for comparing them with existing traces. Off line mode can be also amenable to listeners, e.g. the property access could trigger notifications.

[arcanefoam]

First version in place. Need to use it in current implementation and fisx accordingly.

[arcanefoam]

Need more work on this to make it more like the IModel interface and build on some of the concepts proven like load/store/dispose and use the properties to allow different trace persistance mechanisms.