Improve the detection of schema changes and add tests

[tmadlener]

BEGINRELEASENOTES

• Improve the schema changes that podio_schema_evolution.py detects and reports on
  • Addition / removal of VectorMembers, OneToOneRelations and OneToManyRelations
• Make the script exit with a non-zero exit code in case an unsupported schema change is detected
• Add a test setup and some tests for testing the schema evolution script and make sure it detects unsupported changes

ENDRELEASENOTES

Fixes #712

• Implement detection of changes to OneToOneRelations
• Implement detection of changes to OneToManyRelations

[tmadlener]

This works for now, but I am not extremely happy with the overall implementation at the moment. At some point there should be some refactoring of the whole detection system and script to make it easier to test and extend.

[tmadlener]

@hegner anything speaking against solving this like this for now and then doing some refactoring later?