Use DataHandles in unpackers

Merged Alex Pearce requested to merge apearce-unpacker-datahandles into master

This converts a bunch of unpackers to use DataHandles for defining input and output locations. The motivation is for simpler usage in the new scheduler, where I've been using the unpackers in an HLT2 prototype that takes the Brunel reconstruction from Upgrade MC files.

Most of the changes are purely mechanical. The biggest change is to remove support for separate packed weights vectors for RecVertices. This is an old packing scheme, and I figure we won't use this in the future.

I tried converting them to functional but my first attempt ended up with null pointers in some SmartRefs. I'm not sure how valuable it would be to investigate further. If people think it's worthwhile, I can dig deeper.

/cc @jonrob

Edited by Marco Cattaneo

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