Add qchem vibronic transition operation

[soranjh]

This PR adds the function VibronicTransition to the apps.qchem.vibronic module. This function generates a custom sf operation for applying the Doktorov operator on a given state.

[codecov]

Codecov Report

Merging #451 into master will decrease coverage by 1.00%.
The diff coverage is 99.40%.

@@            Coverage Diff             @@
##           master     #451      +/-   ##
==========================================
- Coverage   97.84%   96.83%   -1.01%     
==========================================
  Files          58       70      +12     
  Lines        6784     7086     +302     
==========================================
+ Hits         6638     6862     +224     
- Misses        146      224      +78     

Impacted Files	Coverage Δ
strawberryfields/apps/clique.py	100.00% <ø> (ø)
strawberryfields/apps/plot.py	100.00% <ø> (ø)
strawberryfields/apps/train/embed.py	100.00% <ø> (ø)
...awberryfields/backends/gaussianbackend/__init__.py	100.00% <ø> (ø)
strawberryfields/backends/gaussianbackend/ops.py	100.00% <ø> (+2.70%)	⬆️
strawberryfields/backends/shared_ops.py	96.11% <ø> (-0.98%)	⬇️
strawberryfields/engine.py	95.45% <ø> (+0.55%)	⬆️
strawberryfields/io.py	97.70% <ø> (-1.13%)	⬇️
strawberryfields/ops.py	98.84% <ø> (-0.18%)	⬇️
strawberryfields/parameters.py	99.06% <ø> (+0.03%)	⬆️
... and 75 more

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[trbromley]

Thanks @soranjh, looks great!

[agran2018]

Thanks @soranjh. Just left one question on the unit test.
Another question, why not to call this function Doktorov?. I noticed in the test that there are two "vibronic" prefixes vibronic.VibronicTransition(U1, r, U2, alpha) | q.

[soranjh]

Thanks @soranjh. Just left one question on the unit test.
Another question, why not to call this function Doktorov?. I noticed in the test that there are two "vibronic" prefixes vibronic.VibronicTransition(U1, r, U2, alpha) | q.

We also thought of Doktorov and it is still an option. The idea is to name the operators in a similar way to the existing sf.ops operators, e.g., MeasureFock. We will make these operations available as qchem.OperatorName in the future.

[ixfoduap]

Should be good to go! I just left a couple of suggestions regarding the docstring