Add vtkdiff submodule and its compilation.

.gitmodules

@@ -10,3 +10,6 @@
 [submodule "ThirdParty/tetgen"]
 	path = ThirdParty/tetgen
 	url = https://github.com/ufz/tetgen.git
+[submodule "vtkdiff"]
+	path = ThirdParty/vtkdiff
+	url = https://github.com/ufz/vtkdiff.git

ThirdParty/CMakeLists.txt

@@ -44,3 +44,9 @@ if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/tetgen/CMakeLists.txt)
 		install (PROGRAMS $<TARGET_FILE:tetgen> DESTINATION bin COMPONENT ogs_extras)
 	endif()
 endif()
+
+# vtkdiff
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/vtkdiff/CMakeLists.txt)
+	add_subdirectory(vtkdiff)
+	install (PROGRAMS $<TARGET_FILE:vtkdiff> DESTINATION bin COMPONENT ogs_extras)
+endif()

vtkdiff @ d8af1935

+Subproject commit d8af1935bc8577c6d2accb2c828524ead107630b

scripts/cmake/SubmoduleSetup.cmake

@@ -6,6 +6,7 @@ if(NOT OGS_ADDITIONAL_SUBMODULES_TO_CHECKOUT)
 endif()
 set(REQUIRED_SUBMODULES
 	ThirdParty/quickcheck
+	ThirdParty/vtkdiff
 	${OGS_ADDITIONAL_SUBMODULES_TO_CHECKOUT}
 )