grav dataset: change phenotype name T125 -> T124

 - also change the corresponding times attribute
 - this makes the times equally spaced
 - I'm not sure what the "T125" is in there when the rest of the times
   are T0, T2, T4, T6, ..., T480, but it is also in the
   [data at the QTL archive](http://qtlarchive.org/db/q?pg=projdetails&proj=moore_2013b)