Better kernel dispatch and add custom _asarray

[chaeyeunpark]

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Context: Currently, we have two different gate implementations (kernels) and some more to be added. As it is possible that one kernel outperforms another kernel only for some number of qubits, it is desirable to have a kernel dispatcher mechanism that takes this information into account. In addition, as with AVX kernels, it is necessary to have an array whose memory is managed by lightning with a proper alignment.

Description of the Change: Lightning now has the KernelMap class which returns the kernels to use for a given number of qubits. The statevector classes now manage their own kernel information accordingly. Moreover, _asarray is implemented.

Benefits: Not huge right now, but with AVX and OpenMP kernels, we should have some advantages.

Possible Drawbacks: Slightly slower dispatch time (1 or 2 more std::unoreder_map accesses), and constructing a statevector becomes slower as it needs to obtain the best kernels for each operation.

Related GitHub Issues:

Still need to fix/add documents.

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Test Report (C++) on Ubuntu

           1 files  ±    0             1 suites  ±0   0s ⏱️ ±0s
       761 tests  -     3         761 ✔️  -     3  0 💤 ±0  0 ❌ ±0 
228 963 runs  +395  228 963 ✔️ +395  0 💤 ±0  0 ❌ ±0 

Results for commit 0eced01. ± Comparison against base commit fab55ea.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

Merging #245 (9528481) into master (fab55ea) will decrease coverage by 3.64%.
The diff coverage is 92.86%.

❗ Current head 9528481 differs from pull request most recent head 6e35950. Consider uploading reports for the commit 6e35950 to get more accurate results

@@            Coverage Diff             @@
##           master     #245      +/-   ##
==========================================
- Coverage   99.19%   95.55%   -3.65%     
==========================================
  Files          28       34       +6     
  Lines        3105     2699     -406     
==========================================
- Hits         3080     2579     -501     
- Misses         25      120      +95     

Impacted Files	Coverage Δ
...ennylane_lightning/src/gates/OpToMemberFuncPtr.hpp	100.00% <ø> (ø)
...lightning/src/gates/cpu_kernels/PauliGenerator.hpp	100.00% <ø> (ø)
pennylane_lightning/src/util/ConstantUtil.hpp	92.30% <ø> (-7.70%)	⬇️
pennylane_lightning/src/util/LinearAlgebra.hpp	80.59% <ø> (-16.90%)	⬇️
pennylane_lightning/src/util/Memory.hpp	63.63% <63.63%> (ø)
...nnylane_lightning/src/simulator/CPUMemoryModel.hpp	66.66% <66.66%> (ø)
...lane_lightning/src/simulator/DynamicDispatcher.hpp	78.94% <75.00%> (ø)
...nylane_lightning/src/simulator/StateVectorBase.hpp	84.61% <78.37%> (-15.39%)	⬇️
pennylane_lightning/src/simulator/DispatchKeys.hpp	85.71% <85.71%> (ø)
pennylane_lightning/src/simulator/KernelMap.hpp	90.27% <90.27%> (ø)
... and 43 more

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[chaeyeunpark]

Continue at #291