Resolve "ALICE_2016_I1507157 Analysis bug with Lambda baryons"

Closes #300 (closed). Draft merge request to make a branch. Will be updated. I totally promise! Don't know whether @agbuckley or @20DM gets here first. Please have a look at the code before approving. My Rivet-fu feels a bit rusty, and I am not sure if better solutions for eg YODA hacks have been introduced.