[WIP] Schema evolution code generation

[hegner]

BEGINRELEASENOTES

• Thank you for writing the text to appear in the release notes. It will show up
  exactly as it appears between the two bold lines
• ...

ENDRELEASENOTES

Based on #430

• Needs [WIP] Moving PODIO to using rootcling instead of reflex #460

[tmadlener]

Based on #430

Just for clarification from my side: This means we still keep 430 as it is and merge it separately, or we close that and keep this one for the feature?

[hegner]

Since I am fixing bugs in #430, I think we better close it. I previously thought functionality in there was stable.

[tmadlener]

Rebased this onto #460. For me locally there are a few test issues with the datamodel storage roundtrip tests, because they now trip over the fact that for the schema evolution tests we generate "old" Data.h headers into the same directory. This makes the final diff step in those tests fail.

I don't have a real fix for that yet, but I would simply wait for now to see how the final solution to schema evolution looks like and then fix those tests afterwards.

[hegner]

Since based on #430 , this development will not be integrated.