Unclear behavior around version container when TDs are generated from TMs

[egekorkan]

Coming from eclipse-editdor/editdor#137, it is not clear what should happen to the version value when a TD is generated from a TM. I think the https://w3c.github.io/wot-thing-description/#thing-model-td-generation needs to be extended with a behavior.

• If there is no version field in a TM, generated TD should have no version. If the TD producer wants it, they can add it.
• If there is version field in a TM, it doesn't have anything mandatory. However, it cannot have instance. Typically, it has model. In that case, the behavior is unclear. The TD generator can put a value for instance or the version object can be removed. One of these two MUST happen, otherwise the generated TD is invalid as the instance keyword is mandatory for version.