OT t0 time tools provoke FP auditor into crashing

Gaudi::Math::ValueWithError's constructor and error methods are not transparent to quiet NaNs, since they do arithmetic on the constructor's arguments. Until we've figured out a good way to deal with that that impacts nobody else, we change the type returned by the OT track t0 tools (with a type alias) to something much simpler that is transparent to NaNs. Fix one place where a NaN produced by the OT track t0 tools confused the packing, while we're at it.