Split off STS Unpacker from global Unpacker class.
Seperates the STS Unpacker from the global Unpacker class. This should make the transition to the GPU Unpacker easier and provide a bit more flexibility when running unpackers.
Once this MR lands, I'll also port the other unpackers to this scheme. To this I've introduced a generic CommonUnpacker
class that acts as a baseclass
for (CPU) unpacker and contains the steering logic for unpacking microslices.