WIP: ATLASRECTS-5150: Fix b-tagging shallow copies to have original object links

As reported in https://its.cern.ch/jira/browse/ATLASRECTS-5150, the b-tagging shallow copies do not copy the original object link, which means MET cannot be calculated. This is a small fix to automate this link, which users currently have to do themselves. The code should work even if the user sets the link themselves later, so this will be forwards/backwards compatible in case people have workarounds. The smart slimming for this container had to be extended to include the link as well.

Marcus is currently testing this in his analysis code, and I'll remove the WIP once he confirms things work.