[T] Add xdmfdiff as submodule to ogs

.gitmodules

@@ -46,3 +46,6 @@
 [submodule "ThirdParty/spdlog"]
 	path = ThirdParty/spdlog
 	url = https://github.com/gabime/spdlog.git
+[submodule "ThirdParty/xdmfdiff"]
+	path = ThirdParty/xdmfdiff
+	url = https://gitlab.opengeosys.org/ogs/xdmfdiff.git

xdmfdiff @ 428760e7

+Subproject commit 428760e717acef51fdedcc2bf3e56474c1b971ef

scripts/cmake/SubmoduleSetup.cmake

@@ -21,7 +21,7 @@ set(REQUIRED_SUBMODULES
     ${OGS_ADDITIONAL_SUBMODULES_TO_CHECKOUT}
 )
 if(BUILD_TESTING)
-    list(APPEND REQUIRED_SUBMODULES ThirdParty/vtkdiff)
+    list(APPEND REQUIRED_SUBMODULES ThirdParty/vtkdiff ThirdParty/xdmfdiff)
 endif()
 if(OGS_BUILD_UTILS)
     # Required by the partmesh tool, which is build with utils only.