Implemented a 'republish' function #138.

nomad/api/upload.py

nomad/client/migration.py

nomad/coe_repo/upload.py

nomad/migration.py

nomad/processing/base.py

nomad/processing/data.py

tests/processing/test_base.py

tests/processing/test_data.py

tests/test_api.py

tests/test_coe_repo.py

tests/test_files.py

tests/test_migration.py