Add HistogramArchiver
This is an AnalysisAlgorithm that saves histograms generated by HistogramAlgorithm. Similarly to DataArchiver, we have to somehow pass an output directory into the class.
This is mostly for discussion in relation to !437 (closed) !435 (closed).
NB as it is, the LoopStatus is not embedded into the file name, so multiple files with the same name are written.
If this goes ahead, there are many possibilities for configuring which files to save based on LoopStatus, histogram names, FE, etc.