Added configurability of domain specific metadata.

nomad/client/local.py

nomad/coe_repo/calc.py

nomad/coe_repo/upload.py

nomad/config.py

nomad/datamodel/__init__.py

nomad/datamodel/base.py

nomad/datamodel/dft.py

nomad/migration.py

nomad/normalizing/__init__.py

nomad/normalizing/repository.py

nomad/normalizing/system.py

nomad/parsing/backend.py

nomad/processing/data.py

nomad/search.py

tests/test_api.py

tests/test_coe_repo.py

tests/test_datamodel.py

tests/test_normalizing.py

tests/test_search.py