Change CI using ctest and little refactorings.

.gitlab-ci.yml

CMake/VirtualFluidsMacros.cmake

CMakeLists.txt

gpu.cmake

src/gpu/VirtualFluids_GPU/Restart/RestartObject.cpp

src/gpu/VirtualFluids_GPU/Restart/RestartObject.h

src/gpu/VirtualFluids_GPU/Restart/RestartObjectTests.cpp