hep: consider implementing hepfile format

[sbinet]

we should consider implementing the HDF5-based file format hepfile from:

• https://github.com/mattbellis/hepfile
• slides from DIANA-HEP

(if it gains traction. also, it would be better if we had a pure-Go implementation of hdf5, either from go-hdf5 or from translating the original HDF5 library with ccgo)