B2tautau 3pimu + eta'tautau 6pi
Added 3 new decfiles. All files are properly tested.
I thought I added the eta' one in a previous merges. But it's not there. I'm not sure what happened. It was approved but maybe it was not merged. Anyway I readied it here. In this case if you accept this the other MR with only the eta' can be removed.