Add magfield and test data as git submdules #1678

Job	Run time
build (macos-latest)	3m 11s
download_test_data	5s
build (ubuntu-latest)	2m 37s
java_matrix	2s
unit_tests (21)	2m 46s
unit_tests (25)	3m 1s
generate_documentation	1m 2s
spotbugs	5m 0s
test_decoder	3m 5s
test_clara	4m 15s
dependency_analysis	1m 3s
test_run-groovy	57s
test_coatjava (ubuntu-latest, eb-ep)	1m 53s
test_coatjava (ubuntu-latest, eb-epc)	2m 14s
test_coatjava (ubuntu-latest, eb-eftpi)	1m 56s
test_coatjava (ubuntu-latest, kpp)	3m 0s
test_coatjava (macos-latest, eb-ep, ./run-eb-tests.sh -100 electronproton)	3m 14s
test_coatjava (ubuntu-latest, eb-eg)	2m 29s
test_coatjava (ubuntu-latest, eb-enc)	1m 49s
release_build	-1s
deploy_web_pages	0s
final	2s
	43m 40s

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