#1 Schema evolution

[Robert]

Currently, only trivial schema evolution can be handled automatically, such as: attribute addition, removal. There are a lot of other changes, which are not handled, like: attribute rename, attribute move to other sub-/super-class, default value for new attribute. Basically, there should be a "transformation language", which should be able to describe such changes.

There should be a more controlled mechanism to update the schema too to avoid schema "trashing", when two concurrent nodes try each to set it's own schema.