Ability to only pass parts of the blob to a module

Summary

The ability to pass just parts of the Blob to an attached module would simplify many pipeline designs. Especially it would make it possible to easily attach e.g. multiple Sinks to the pipeline, each writing to a different file/stream (as needed by @mmoser).

pipe.attach(HDF5Sink, filename='a.h5', blob_keys=['A'])
pipe.attach(HDF5Sink, filename='b.h5', blob_keys=['B'])

Would for example write A to a.h5 and B to b.h5 automatically since other keys are hidden (the HDF5Sink automatically saves everything with an h5loc attribute into the output file).

What is the current workaround to achieve the same functionality?

Nothing acceptable.