bundle2: make sure unbundled part param are read-only My old uncle Robert once trusted an API user. We never saw him again.

bundle2: introduce an ``_initparams`` method The handling of parameters will become much more sophisticated in the coming changesets. So we extract the logic in a function to not pollute the generic logic.

bundle2: forbid duplicate parameter keys No rules were specified about parameter key uniqueness. We document that keys should be unique and document it. This opens the way to a more friendly (read dictionary like) way to access value of parameters in the code.