slot.py: Improved an assert message.
OpStackWriter: Support multi-channel images.
OpInputDataReader: Check that file exists before opening with vigra.impex, and raise the correct exception if not.
export operators:
- Added a 'Transaction' slot that can be used to set all slots atomically (avoid multiple calls to setupOutputs)
- Refactored supported formats list
- Implemented a FormatSelectionIsValid slot