rbind adapted to new scheme.  Would like to change semantics.
rbind is now aware of control, form, extra and prepared.
Removed pad.

Also factored some common code out of if/else.  This made it easier to add in
the new list elements.  The resulting return values will have list
elements in a different order than before.

Documentation is now more explicit about the (bizarre) semantics.
Eliminate detailed documentation of most values copied from x to result.
cbind: factor out some common code and drop pad from results
rbind: use z for return value.
r was already used for somthing else.  Also I forgot to update the variable
name used in the last couple of lines.
cbind: documentation adjustments.
cbind fully(?) on new scheme.  Use NA_character, not ''
for values we really don't know.

As advertised in the comments, I basically punted on $prepared.
It is quite redundant with the information at the mids level anyway.
rbind: fix typo
ibind: moved to new scheme.
ibind has the semantics I was looking for in rbind.

Also partially cleaned up the end of the text of error messages.
mice.mids move to new scheme.  But a bit more restrictive.
The mids object must be from a run of at least one iteration; this
did not used to be true and is not desirable.

Also noticed loggedEvents was not in def of mids.
Added loggedEvents as slot of mids.
This reflects existing useage; e.g., mice created a loggedEvents member of
the class.

Also noted the unfortunate duplication of mids and prepared.
kill off padModel() entirely.