Improvement: Implement FetchContent #321

Job	Run time
fine_grained_trigger	9s
t8code_doxygen_check / check_documentation	1m 8s
t8code_tarball / build_tarball	1m 0s
t8code_w_shipped_submodules_tests / t8code_cmake_tests	4m 16s
t8code_w_clang_and_ompi_tests (ON, Debug) / t8code_cmake_tests	1m 40s
preparation (OFF) / cmake_preparation	1m 54s
preparation (ON) / cmake_preparation	1m 42s
t8code_tarball / test_tarball	10m 45s
sc_p4est_tests (OFF) / sc_p4est_cmake_tests	1m 37s
t8code_api_tests (ON, Release) / t8code_cmake_tests	1m 48s
sc_p4est_tests (ON) / sc_p4est_cmake_tests	1m 26s
t8code_api_tests (ON, Debug) / t8code_cmake_tests	1m 40s
t8code_code_coverage / generate-codecov	1m 47s
t8code_linkage_tests (OFF, Release) / t8code_cmake_tests	1m 31s
t8code_linkage_tests (ON, Release) / t8code_cmake_tests	1m 54s
t8code_tests (ON, Debug) / t8code_cmake_tests	1m 55s
t8code_linkage_tests (ON, Debug) / t8code_cmake_tests	1m 33s
t8code_tests (OFF, Release) / t8code_cmake_tests	1m 58s
t8code_tests (ON, Release) / t8code_cmake_tests	1m 44s
t8code_tests (OFF, Debug) / t8code_cmake_tests	1m 33s
t8code_linkage_tests (OFF, Debug) / t8code_cmake_tests	1m 28s
t8code_valgrind_tests	-1s
	44m 27s

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