Fix a bug in the generator of DoubleExcitationPlus

[maliasadi]

Context:
Fix a bug in applyGeneratorDoubleExcitationPlus.

[github-actions]

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[github-actions]

Test Report (C++) on Ubuntu

           1 files  ±    0             1 suites  ±0   1s ⏱️ ±0s
       862 tests ±    0         862 ✔️ ±    0  0 💤 ±0  0 ❌ ±0 
228 704 runs  +180  228 704 ✔️ +180  0 💤 ±0  0 ❌ ±0 

Results for commit b48d04a. ± Comparison against base commit a113c30.

♻️ This comment has been updated with latest results.

[AmintorDusko]

Hi @maliasadi. Nice catch!
I wonder why the unit tests didn't get this bug.

[maliasadi]

Hi @maliasadi. Nice catch! I wonder why the unit tests didn't get this bug.

Hmm! It required an improvement in generator tests to catch this bug.

[chaeyeunpark]

Hi @maliasadi. Nice catch! I wonder why the unit tests didn't get this bug.

This bug does not appear when the number of qubits is 4 (the minimum number of wires required by DoubleExcitationPlus). As we did not have 4 qubit operators before, I just used the fixed number of qubits = 4 in generator test suits. I now update it and test up to 8 qubits.

[AmintorDusko]

Happy to approve. Once again, nice catch!

[codecov]

Codecov Report

Merging #298 (b48d04a) into master (a113c30) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #298   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          30       30           
  Lines        3300     3300           
=======================================
  Hits         3281     3281           
  Misses         19       19           

Impacted Files	Coverage Δ
pennylane_lightning/_version.py	100.00% <100.00%> (ø)
...src/gates/cpu_kernels/QChemGateImplementations.hpp	100.00% <100.00%> (ø)

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