Add magfield and test data as git submdules #1675

Job	Run time
java_matrix	3s
build (ubuntu-latest)	3m 0s
download_test_data	1m 13s
build (macos-latest)	3m 31s
unit_tests (21)	3m 18s
unit_tests (25)	3m 26s
test_decoder	3m 32s
spotbugs	6m 52s
test_clara	5m 0s
dependency_analysis	1m 13s
test_coatjava (ubuntu-latest, eb-eg)	1m 0s
test_coatjava (ubuntu-latest, eb-enc)	1m 10s
test_coatjava (ubuntu-latest, eb-eftpi)	1m 5s
test_run-groovy	1m 15s
test_coatjava (ubuntu-latest, eb-ep)	1m 11s
test_coatjava (ubuntu-latest, eb-epc)	1m 7s
test_coatjava (ubuntu-latest, kpp)	58s
test_coatjava (macos-latest, eb-ep, ./run-eb-tests.sh -100 electronproton)	1m 14s
release_build	0s
generate_documentation	1m 10s
deploy_web_pages	-1s
final	0s
	41m 17s

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