Fix warning on r_KLU_rocsolverrf_asym6x6

[nkoukpaizan]

Description

There were some integer conversion warnings on r_KLU_rocsolverrf_asym6x6 due to the use of std::vector, which is indexed differently from Re::Solve storage.

ReSolve/examples/experimental/r_KLU_rocsolverrf_asym6x6.cpp:52:37: warning: implicit conversion changes signedness: 'int' to 'size_type' (aka 'unsigned long') [-Wsign-conversion]
   52 |   std::vector<int>    csr_row_ptr(n + 1, 0);

Proposed changes

Use static_cast to ensure the proper integer types are used to index std::vector and to instantiate Re::Solve matrices and vectors.

Checklist

• All tests pass. Code tested on
  • CPU backend
  • CUDA backend
  • HIP backend
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• [N/A] The new code follows Re::Solve style guidelines.
• [N/A] There are unit tests for the new code.
• [N/A] The new code is documented.
• The feature branch is rebased with respect to the target branch.

Further comments

We could consider avoiding the use of std::vector here altogether. I'm for keeping it, as this is an example, and users should be free to use whatever storage they wish.

[shakedregev]

Approving and updating documentation.