Changing approach to use parent entry and mainfile_key

nomad/app/v1/routers/uploads.py

nomad/cli/admin/migrate.py

nomad/client/processing.py

nomad/datamodel/datamodel.py

nomad/files.py

nomad/parsing/parser.py

nomad/parsing/parsers.py

nomad/processing/data.py

tests/app/v1/routers/common.py

tests/app/v1/routers/test_entries.py

tests/app/v1/routers/test_uploads.py

tests/conftest.py

tests/parsing/test_parsing.py

tests/processing/test_data.py

tests/test_files.py