bundle2: introduce a ``params`` dictionary on unbundled parts This exposes all parameters the part received into a ``part.params`` dictionary. This should be much easier to use. This dictionary itself does not expose the mandatory or advisory aspect of parameters, but no current users of bundle2 actually enforce any of this logic. Coming changesets will improve this aspect.

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.

bundle2: update part creators to ``addparam`` when relevant In some cases, the use of ``addparam`` makes code much clearer.

bundle2: introduce a ``addparam`` method on part We make it easier to add new parameters after the part creation. As for the ``data`` attribute we make sure the part generation has not begun yet.