Add missing SYCL backend operators and kernels to allow automatic compilation

Issue/feature addressed

Add SYCL backend for

• AddTraceIntegral
• DirBndCond
• NeuBndCond
• RobBndCond
• DiagPrecon
• LoopExecution
• MathKernel

Re-enable ADD_OPERATOR for SYCL backend in library/Operators/CMakeLists.txt

Proposed solution

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

Nektar++ can be compiled with SYCL using

cmake .. -DNEKTAR_BUILD_REDESIGN=ON \
         -DNEKTAR_ENABLE_SYCL=ON \
         -DCMAKE_CXX_COMPILER="/apps/llvm/sycl-2024-06-06/bin/clang-19" \
         -DCMAKE_CXX_FLAGS="-fsycl -fsycl-targets=nvptx64-nvidia-cuda" \
         -DNEKTAR_TEST_ALL=ON

Checklist

• Functions and classes, or changes to them, are documented.
• [ ] User guide/documentation is updated.
• [ ] Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).