Various migration related improvements.

nomad/datamodel.py

nomad/files.py

nomad/migration.py

nomad/normalizing/normalizer.py

nomad/normalizing/repository.py

nomad/normalizing/system.py

nomad/parsing/backend.py

nomad/processing/base.py

nomad/search.py

nomad/utils.py

tests/test_api.py

tests/test_search.py