Define a first-class abstraction for the DT Model ?

[samubura]

In WLDT currently, the model of the DT is implemented through the shadowing, function.
This gives a bit too much responsibility to the shadowing function which in theory should be responsible only for mapping the physical updates to the Digital updates.

Any form of decision-making, applying changes on the PA, or computation that concerns aggregated properties and so on could be delegated to the model of the DT, to which the shadowing function passes events.

This may create less confusion for library adopters as well.

This opens up many patterns:

• can there be multiple models?
• what is the purpose of the shadowing function then?
• do we provide a default shadowing process that simply propagate every event while delegating them also to the model?