Use LoopStatus::toString() inside HistoXd::toFile()

Once !1017 (merged) is merged we can use LoopStatus::toString() to create histo filenames instead of having the histogram class do it.

One thing to note is that in the current implementation of !1017 (merged), toString() uses the dash as a separator instead of an underscore. This is in keeping with the dashes used inside many analyses (and thus many output files), but the default behaviour of the HistoXd classes is generally to use an underscore instead.