Add CMake support for execution space specific but implementation generic operators

Issue/feature addressed

Some operators do not have a StdMat or a SumFac specific implementation. The implementation should be "generic".

Proposed solution

Update the Operators/CMakeList.txt to recognize a *Generic.hpp header file.

Also update to use SumFac implementation for Kokkos backend.

Also simplify operator defaulting.

Implementation

Tests

Suggested reviewers

Notes

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).