Removal of temporary histogram files

[aebid]

In the workflow:
HistTupleProducer -> HistFromNtupleProducer -> HistMerger -> HistPlot

There are several temporary files made that are not cleaned up.

First, in MistHerger there is a temporary output folder /version/era/merged/variable/tmp/ that holds the temporary files that is not cleaned.

Second, after HistMerger the previous HistFromNtupleProducer is pointless. There is no loss in knowledge between these steps and HistFromNtupleProducer should be treated as temporary files and removed after success