graph.py: Removed destination parameter from Slot.get().  Preferred method for providing destination is Request.writeInto()
graph.py: Also, use perfect forwarding for Operator.__new__
Avoid unecessary lambdas when using map()
BlockwiseFileset: Added exportSubset() function and related helpers.
BlockwiseFileset: Added bin/exportRoi.py for creating cutout volumes from a blockwise fileset.
graph.py: Added an assertion in Slot.connect() to prevent connection between slots with incompatible stypes.