WIP: Cherry pick HDF5Utils to 21.9

This is a cherry pick of code from master, to the extent that it was possible. Some other packages that were merged more "manually" (i.e. by copying files) caused conflicts so I had to resolve a bit by hand.

The commits add HDF5Utils and should give something which is identical to that in master.

Note that this also requires turning on HDF5 in AthenaExternals. This is a first: we don't currently build HDF5 in the 21 releases of Athena (only AthAnalysis and AnalysisBase), and in release 22 the version that comes with LCG is modern enough.