feat: allow to rebuild a model

Spoon currently limit the building of a model. Basically once the model is built, it cannot be built again or we get an exception "Model already built".

If I remember well, this has been historically done on purpose to avoid inconsistencies in some cache in the factories. Am I right here @monperrus ?

Anyway I don't think we should keep this behaviour: as a tool transformation, it's really a pain to not be able to rebuild if some new sources are added or if sources have changed.

I don't see any rationale against removing this check and wiping all caches and information before building the model, in order to allow building multiple time a model. The goal is not to be incremental here as we already have a launcher dedicated to that, but just to allow multiple builds with the same launcher.

WDYT?