Sherpa 2.2.10 ttbb: add muF/4 variation weight
As just discussed in PMG Top Modelling this commit adds the 0.25muF scale variation just in case we decide to move to a central scale of 0.5muF in the recently submitted Sherpa 2.2.10 ttbb JOs (ATLMCPROD-8900).
@jkretz and @cgutscho should decide whether they managed to hold the production of the sample and hence want to include this update, or whether the production has already commenced, in which case we won't do this update anymore.
[skip modfiles]