Add t8_element_is_ghost function #500

Job	Run time
t8code_w_shipped_submodules_tests / t8code_cmake_tests	6m 40s
preparation (OFF) / cmake_preparation	1m 9s
preparation (ON) / cmake_preparation	1m 9s
t8code_api_tests (ON, Release) / t8code_cmake_tests	4m 4s
t8code_tests (OFF, Release) / t8code_cmake_tests	2m 45s
t8code_tests (OFF, Debug) / t8code_cmake_tests	3m 51s
t8code_api_tests (ON, Debug) / t8code_cmake_tests	6m 18s
t8code_tests (ON, Debug) / t8code_cmake_tests	6m 30s
t8code_tests (ON, Release) / t8code_cmake_tests	3m 51s
sc_p4est_tests (ON) / sc_p4est_cmake_tests	3m 29s
t8code_linkage_tests (OFF, Debug) / t8code_cmake_tests	11m 25s
sc_p4est_tests (OFF) / sc_p4est_cmake_tests	3m 42s
t8code_linkage_tests (OFF, Release) / t8code_cmake_tests	7m 39s
t8code_linkage_tests (ON, Debug) / t8code_cmake_tests	17m 38s
t8code_linkage_tests (ON, Release) / t8code_cmake_tests	10m 19s
	1h 30m 29s

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