[WIP] Add automatic Frame legacy read

[hegner]

BEGINRELEASENOTES

• Transparent handling when reading legacy PODIO files

ENDRELEASENOTES

This PR is WIP. It seems to suffer from a bug in underlying libs which need to be addressed first.

[hegner]

@andresailer - after the discussion yesterday. There is a bug to likely fix externally first, but this should be the code needed on FWK side. We can wait for this or merge #100 independently.

[hegner]

@tmadlener - working on this showed an interesting incident, why moving to frames was the right thing. We have https://github.com/key4hep/k4FWCore/blob/master/k4FWCore/components/PodioOutput.cpp#L108 that uses the old schema for the collection info. Even though the PODIO used does the newer schema. Thus legacy files written with PodioOutput are not compatible with plain PODIO. Unfortunately we may need to put the error handling into PODIO itself into the ROOTLegacyReader.

[tmadlener]

Could the old schema version for collection info be the root cause for #95?

Putting the handling of that into the ROOTLegacyReader sounds fine to me.

[hegner]

Yes, that could be an explanation for #95.

[andresailer]

This needs a rebase and conflict resolution if still valid?

[tmadlener]

Rebased onto the latest master and things should be compiling again. As discussed during the last key4hep meeting, this should be able to read legacy files as written by the podio::ROOTWriter, but will most likely fail to read legacy files produced by k4FWCore. In order to make that work it is still necessary to repackage the current (k4FWCore) legacy input system to produce Frame(Data).

[jmcarcell]

Can we close this? We are already going to have files pre EDM4hep 1.0 (these will be the "legacy" ones) and after EDM4hep 1.0

[tmadlener]

I think we can close this for now and come back if we actually need this functionality in the future.