Added basic ELN search capabilities. #826

examples/data/eln/example-data.archive.yaml

examples/data/eln/example-schema.archive.yaml

gui/src/components/search/FilterRegistry.js

gui/src/components/search/input/InputField.js

gui/src/components/search/menus/FilterMainMenu.js

gui/src/components/search/menus/FilterSubMenuELN.js

nomad/datamodel/data.py

nomad/datamodel/metainfo/eln/__init__.py

nomad/datamodel/results.py