Add magfield and test data as git submdules #1674

Job	Run time
build (macos-latest)	2m 50s
build (ubuntu-latest)	2m 42s
download_test_data	11m 21s
java_matrix	4s
unit_tests (21)	3m 16s
unit_tests (25)	2m 47s
spotbugs	6m 56s
test_run-groovy	46s
dependency_analysis	1m 12s
test_coatjava (ubuntu-latest, kpp)	59s
test_coatjava (ubuntu-latest, eb-eftpi)	1m 1s
test_coatjava (macos-latest, eb-ep, ./run-eb-tests.sh -100 electronproton)	1m 1s
test_coatjava (ubuntu-latest, eb-eg)	1m 12s
test_coatjava (ubuntu-latest, eb-ep)	1m 9s
test_coatjava (ubuntu-latest, eb-enc)	1m 1s
test_coatjava (ubuntu-latest, eb-epc)	1m 2s
release_build	0s
generate_documentation	1m 1s
deploy_web_pages	0s
final	0s
test_clara	0s
test_decoder	0s
	40m 20s

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