Filename sometimes defined in format section

[pnuu]

Sometimes filename is defined in formats section after the first composite is saved, and this caused crashes when filename was also provided in the scn.save_dataset() call. I'm not certain if this happens due to some configuration problem, or if plist_iter sees the format sections connected.
This was "fixed" in #25 by removing the possible filename from the format dictionary, but the real cause should be found.

[pnuu]

Superseded by #99