Deprecation of edm::OwnVector

[makortel]

• Open an issue in CMSSW GitHub #633
• Mark edm::OwnVector as CMS_DEPRECATED #634
  • Experiment a bit what kind of placement for the custom attribute makes most sense (whole class, constructor, something else?)
  • Ask Shahzad to revive CMSDEPRECATED IB? Maybe consider enabling some -Wdeprecated* there as well?
• Collect all uses of edm::OwnVector in the issue #599
• Remove unnecessary uses of OwnVector #600
• Remove or replace OwnVector<SiStripMatchedRecHit2D> #827
• Migrate OwnVector<BaseTrackerRecHit> to vector<unique_ptr<BaseTrackerRecHit>> #665
• Migrate OwnVector<TrackingRegion> to vector<unique_ptr<TrackingRegion>> #835
• Migrate many cases of edm::OwnVector to std::vector #666
  • This will result in data format backwards incompatibilities. Need to understand when we can deploy. E.g. do we need to merge first in DEVEL IB, and when 14_0_X opens we merge all those from DEVEL to master?
• Develop a new version of OwnVector that internally uses std::vector<std::variant<...>>
  • Need to figure out a good name
• Migrate all genuine uses of edm::OwnVector to the variant-based new `OwnVector
  • List in edm::OwnVector is deprecated cmssw#42734 (comment)
  • Also here need to think of the deployment strategy
  • Need ROOT to support vector<variant<...>> in TTree first