Revise `m_to_dict` serialization, simplify logic.

gui/tests/artifacts.js

nomad/datamodel/data.py

nomad/datamodel/datamodel.py

nomad/datamodel/metainfo/annotations.py

nomad/metainfo/__init__.py

nomad/metainfo/data_type.py

nomad/metainfo/elasticsearch_extension.py

nomad/metainfo/metainfo.py

nomad/metainfo/util.py

tests/metainfo/__init__.py

tests/metainfo/test_metainfo.py

tests/metainfo/test_quantities.py

tests/metainfo/test_references.py

tests/processing/test_data.py