Previously, we could not have a setter function for base.HasKind.kind due to the fact, that if the kind of a Submodel changed, we would have to recursivly adapt all the contained SubmodelElement.kind.

With version 3.0 of the specification, SubmodelElement.kind was removed, so therefore there's no reason now not to offer a setter for base.HasKind.kind.

We should note a warning at base.HasKind though, if future versions of the specification add `HasKind somewhere else again, since that would reintroduce the problem of setting it.

This was found due to the question in #300.