Slot: Assert that only input slots can have default values.
Slot.setValue(): Use == instead of 'is' to determine if the value is different.
This reverts commit a43395ecc93f34774ccf3d0d745fc1ccff14d6d8.  See code comments for why this is necessary.
Slot.configured() always returns True if the slot is optional.
Added OpTransposeSlots
OperatorWrapper: Assert that all slot names given in the constructor args are valid.
Slot: Memoized real_operator to avoid some function calls.
Slot.setValue() catches ValueErrors when comparing via __eq__.
Slot.setValue(): Different types doesn't imply inequality unless one of the types is a VigraArray
MetaDict: Added missing import.
Slot: When resize()ing a multislot, connect the subslots AFTER the size change has been propagated to upstream/downstream partners.
Fixes a bug that can occur in tricky diamond-connection patterns in which subslots would not end up with all the partners that the parent multislot has.
Slot: Propagate UN-ready status through the complete graph before propagating the rest of the meta-state.
This ensures that operators that are about to be totally unconfigured don't ask for upstream values during graph setup.
When checking for drange, check if non-None instead of checking for the existance of the key.
OpH5WriterBigDataset: Support datasets with no channel axis.
OpFilterLabels: Support 32-bit machines.
OrderedSignal.unsubscribe() more efficient by using OrderedDict
optimization: ready() uses generator expression, which short-circuits
Rewrote MetaDict to subclass defaultdict.

This speeds up and simplifies __getattr__ and __setattr__. This
optimization is useful for graphs with many lanes, because meta
information is accessed so often.
tests: Use tmp dir for test npy data.
clarified the meaning of parent and graph in class Operator
fix BIG bug in orderedsignal.py