Add SPVM routine for when kokkos not available.

[vincentmr]

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---

Context:
Kokkos & Kokkos-kernels (serial backends) are used only for a single sparse matrix/dense vector operation. This is a

Description of the Change:
Add simple spvm implementation when ENABLE_KOKKOS=OFF.

Benefits:
No need to pull large dependencies Kokkos & Kokkos-kernels. SparseHamiltonian always supported.

Possible Drawbacks:
It could be slower, although the (serial) code is so simple that there is little room for optimization and that is unlikely.

Related GitHub Issues:

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
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[codecov]

Codecov Report

Merging #465 (cd34892) into master (d702423) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #465      +/-   ##
==========================================
+ Coverage   99.82%   99.91%   +0.08%     
==========================================
  Files          50       50              
  Lines        4667     4671       +4     
==========================================
+ Hits         4659     4667       +8     
+ Misses          8        4       -4     

Impacted Files	Coverage Δ
pennylane_lightning/src/simulator/Measures.hpp	99.52% <ø> (ø)
pennylane_lightning/src/util/LinearAlgebra.hpp	100.00% <ø> (ø)
pennylane_lightning/_version.py	100.00% <100.00%> (ø)
pennylane_lightning/lightning_qubit.py	100.00% <100.00%> (+0.85%)	⬆️
pennylane_lightning/src/util/SparseLinAlg.hpp	100.00% <100.00%> (ø)

[mlxd]

Some quick comments for the next week.

[AmintorDusko]

So far looks good to me. I will check in more depth later.

[AmintorDusko]

Hi, @vincentmr. It appears that the CI may have become stuck, so you may want to try re-triggering it.

[mlxd]

Thanks Vincent --- no more thoughts from me. Happy to approve.

[multiphaseCFD]

Nice job @vincentmr !

[vincentmr]

Gist