Merge k4DataSvc and PodioDataSvc

[tmadlener]

As discussed in #100 k4DataSvc is effectively just a very thin wrapper around PodioDataSvc and can be removed. This issue is mainly for keeping track of the removal and for related discussions.

[jmcarcell]

k4DataSvc is used in many repos both in key4hep and other organizations so it's a fair amount of work for not much gain:

• https://github.com/search?q=org%3Akey4hep%20k4DataSvc&type=code
• https://github.com/search?q=org%3AHEP-FCC%20k4DataSvc&type=code
• https://github.com/search?q=org%3Acepc%20k4DataSvc&type=code
  Alternatively we could move everything to k4DataSvc and remove PodioDataSvc since it's only used internally in this repo. I guess it makes sense if you're using key4hep that the name is k4DataSvc

[tmadlener]

Dropping PodioDataSvc in favor of k4DataSvc is fine for me. The main goal is to remove the sort of artificial split.

[tmadlener]

There is another DataSvc FCCDataSvc that effectively does the same as k4DataSvc.

@kjvbrt @BrieucF is that still in use to the best of your knowledge?

[tmadlener]

There might be a technical reason for this split(?) At least simply merging PodioDataSvc into k4DataSvc and moving that to the core k4FWCore library, results in

[warning] library [k4FWCorePlugins] exposes factory [k4DataSvc] which is declared in [libk4FWCore.so] !!

and then subsequently runtime crashes with:

[k4run] : Configurable k4DataSvc not found in module k4FWCore.k4FWCorePluginsConf
Traceback (most recent call last):
  File "/home/tmadlener/work/key4hep/k4FWCore/k4FWCore/scripts/k4run", line 129, in <module>
    load_file(file)
  File "/home/tmadlener/work/key4hep/k4FWCore/k4FWCore/scripts/k4run", line 26, in load_file
    exec(file.read(), {})
  File "<string>", line 22, in <module>
TypeError: 'NoneType' object is not callable

EDIT: That is solvable with putting the DECLARE_COMPONENT into the right place

[BrieucF]

There is another DataSvc FCCDataSvc that effectively does the same as k4DataSvc.

@kjvbrt @BrieucF is that still in use to the best of your knowledge?

Hi Thomas, yes FCCDataSvc is used in some places outside of the Key4hep/HEP-FCC organizations but I can do the modification where I know it is used and advertise the change to the relevant mailing lists to warn people if it is decided to remove FCCDataSvc.

[kjvbrt]

We removed it in some places, but not all, yet.

[tmadlener]

OK. Thanks for checking. We could put in some deprecation warning log message before completely removing it. That would at least make people aware if they are checking their logs.