Enforce policies on ROOT dictionaries

[makortel]

Following up cms-sw/cmssw#45423 (comment)

Preparatory work

• Refactor edmCheckClassVersion #976

Checks in edmDumpClassVersion + checkDictionaryUpdate.py

• persistent=false is allowed only for edm::Wrapper<T> instantiations #983
• Class update should keep all previous class versions+checksums #984
• Class update should add exactly 1 new class to the list of class versions+checksums #985
• Class update should increase class version only by 1 #986
• If a non-template class has a version number set, it should have versions+checksums kept. For a template instantiation, the versions+checksums are not kept. #1006
• New class should start versioning from 3 #1007
• Check that any class whose type may contain aliases does not change its actual type #1008
• Non-dataformat packages may only define transient data formats
• For edm::Wrapper<T>, check that T has a class version set. This would need special treatment for standard types, so might not be worth it in the end.

Canceled (but useful to document here)

• Version number should be updated only when checksum changes No, we've seen a few cases where it has been useful to update the class version without changing the class layout (checksum) in order to trigger io read rules
• Non-template classes should have a class version set Can't easily require this, because we don't require versions for transient classes, and transient classes that are not edm::Wrapper<T> instantiations should not have persistent="false" set.

Checks in edmCheckClassVersion

• Class template instantiations should not have a class version set