Tidy execution model and uniformise template parameter

Issue/feature addressed

Use consistent memory space and execution space template parameter for the Kokkos back-end.

Proposed solution

• Use NektarSpaces::DeviceSpace instead Kokkos::DefaultExecutionSpace::memory_space;
• Use NektarSpaces::HostSpace instead of Kokkos::HostSpace

Implementation

Tests

Suggested reviewers

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

Notes

Please add any other information that could be useful for reviewers.

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).