Add magfield and test data as git submdules #1673

Job	Run time
java_matrix	3s
download_test_data	5s
build (macos-latest)	3m 29s
build (ubuntu-latest)	2m 48s
unit_tests (25)	3m 21s
unit_tests (21)	3m 17s
generate_documentation	1m 8s
test_decoder	3m 4s
test_clara	4m 53s
test_run-groovy	1m 6s
spotbugs	6m 49s
test_coatjava (ubuntu-latest, kpp)	1m 20s
test_coatjava (ubuntu-latest, eb-eg)	1m 15s
test_coatjava (ubuntu-latest, eb-eftpi)	1m 13s
test_coatjava (macos-latest, eb-ep, ./run-eb-tests.sh -100 electronproton)	1m 28s
test_coatjava (ubuntu-latest, eb-enc)	1m 10s
test_coatjava (ubuntu-latest, eb-ep)	1m 18s
test_coatjava (ubuntu-latest, eb-epc)	1m 25s
dependency_analysis	1m 9s
release_build	-1s
deploy_web_pages	0s
final	0s
	40m 20s

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