BUG: Delay npyiter size check when size may change

I know this is a pretty big change in a way, and in a way it isn't... Still needs tests, some will be easy, but since the python interface can't be tricked to test most of this easily (though maybe I could modify it, so that it actually can go through these code paths), I didn't get to it yet (I could trick it if I would allow removing zero sized axes, which is no problem really but a different issue...).

Anyway, thought it may be important enough to put up as PR nevertheless. If someone got some C-tests, I would be happy about that.