Enhance default build type selection #84

Job	Run time
preparation (OFF) / cmake_preparation	1m 31s
preparation (ON) / cmake_preparation	57s
t8code_tests (OFF, Release) / t8code_cmake_tests	2m 53s
t8code_tests (OFF, Debug) / t8code_cmake_tests	4m 11s
t8code_tests (ON, Debug) / t8code_cmake_tests	4m 43s
sc_p4est_tests (OFF) / sc_p4est_cmake_tests	3m 38s
t8code_tests (ON, Release) / t8code_cmake_tests	3m 7s
sc_p4est_tests (ON) / sc_p4est_cmake_tests	3m 13s
t8code_linkage_tests (OFF, Debug) / t8code_cmake_tests	11m 19s
t8code_linkage_tests (ON, Debug) / t8code_cmake_tests	13m 33s
t8code_linkage_tests (OFF, Release) / t8code_cmake_tests	6m 55s
t8code_linkage_tests (ON, Release) / t8code_cmake_tests	8m 5s
	1h 4m 5s

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