Implement CUDA Helmholtz sum-factorization kernels

This is a preliminary implementation of the Helmholtz operator. The Helmholtz operator is build by using a combination of BwdTrans, PhysDeriv, IProductWRTBase, and IProductWRTDerivBas operators. Both a StdMat and a CUDA versions have been implemented. Currently, unity diffusion coefficient is assumed along with lambda = 1. However, provision for non-unity, but constant, diffusion coefficient and a non-unity lambda parameter have been-made. The various sub-operators required to perform the Helmholtz operator are instantiated in the constructor.

Operators/Helmholtz/HelmholtzCUDA.cu

+#include "HelmholtzCUDA.hpp"
+
+namespace Nektar::Operators::detail
+{
+template <>
+std::string OperatorHelmholtzImpl<double, ImplCUDA>::className =
+    GetOperatorFactory<double>().RegisterCreatorFunction(
+        "HelmholtzCUDA", OperatorHelmholtzImpl<double, ImplCUDA>::instantiate,
+        "...");
+}