Implement NullPrecon operator for CUDA

This MR implement the CUDA version of the NullPrecon operator.

It also disable Wpedantic when using CUDA to avoid style of line directive is a GCC extension warning

Operators/NullPrecon/NullPreconCUDA.cu

+#include "NullPreconCUDA.hpp"
+
+namespace Nektar::Operators::detail
+{
+
+// Register implementation with Operator Factory
+template <>
+std::string OperatorNullPreconImpl<double, ImplCUDA>::className =
+    GetOperatorFactory<double>().RegisterCreatorFunction(
+        "NullPreconCUDA", OperatorNullPreconImpl<double, ImplCUDA>::instantiate,
+        "");
+
+} // namespace Nektar::Operators::detail