CI: remove Ubuntu 18.04 and 20.04 CUDA builds

These Ubuntu versions don't seem to support CUDA C++17 by default, so
removing them for now.

Also need to work around an issue that nvcc has with State.hpp.

.github/workflows/ci.yml

@@ -262,8 +262,8 @@ jobs:
       matrix:
         config:
           - os: ubuntu-22.04
-          - os: ubuntu-20.04
-          - os: ubuntu-18.04
+          # - os: ubuntu-20.04 # does not seem to support CUDA C++17
+          # - os: ubuntu-18.04 # does not seem to support CUDA C++17
     env:
       BUILD_TYPE: Release
       CMAKE_FLAGS: -DSPIRIT_UI_USE_IMGUI=OFF -DSPIRIT_UI_CXX_USE_QT=OFF -DSPIRIT_USE_CUDA=ON -DSPIRIT_CUDA_ARCH=sm_70

core/include/data/State.hpp

@@ -38,8 +38,8 @@ struct State
     std::shared_ptr<Engine::Method> method_chain{};
 
     // Timepoint of creation
-    std::chrono::system_clock::time_point datetime_creation{ std::chrono::system_clock::now() };
-    std::string datetime_creation_string{ Utility::Timing::TimePointToString( datetime_creation ) };
+    std::chrono::system_clock::time_point datetime_creation = std::chrono::system_clock::now();
+    std::string datetime_creation_string                    = Utility::Timing::TimePointToString( datetime_creation );
 
     // Config file at creation
     std::string config_file{ "" };