Integrate ROOT and HistFactory input formats

[GraemeWatt]

Integrate the work started by @lukasheinrich on allowing ROOT and HistFactory input formats into the hepdata-converter package.

[lukasheinrich]

if you need support here, let me know.

Cheers,
L

[cranmer]

FYI: This functionality may be highlighted at a workshop at CERN next week.

[eamonnmag]

@lukasheinrich I will do! I'm nearly done with moving all of HEPData on to the new invenio 3. I intend to release the new system next week, all being well (provided that deployment isn't a nightmare).

[lukasheinrich]

we still have this more mid-term plan to extend the number of schemas (currently only one: a table with 1 x and n-y columns). i.e. it would be nice to have a native representation of the histfactory XML (easily translated into yaml) instead of just dumping that as aux. material.