Move vibronic into new qchem module

[trbromley]

This PR moves the existing vibronic module into the new apps.qchem package, providing a single point to access all quantum chemistry functionality in the apps layer.

Positives:

• Users don't have to combine functions from the sample module and the vibronic module, they simply use qchem.vibronic for both encoding the problem and sampling.
• The new location in the qchem module is a natural place for this to go.
• Although a big change from before, it is not a breaking change: users can still interact using the old approach (e.g., apps.vibronic), which we achieve using imports. However, the old approach is no longer mentioned in the documentation and tutorials.

Negatives:

• We don't move the plot.spectrum function.

[codecov[bot]]

Codecov Report

Merging #416 into master will increase coverage by 0.06%. The diff coverage is 99.26%.

@@            Coverage Diff             @@
##           master     #416      +/-   ##
==========================================
+ Coverage   97.84%   97.91%   +0.06%     
==========================================
  Files          58       68      +10     
  Lines        6784     6986     +202     
==========================================
+ Hits         6638     6840     +202     
  Misses        146      146              

Impacted Files	Coverage Δ
strawberryfields/api/result.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%)	:arrow_up:
strawberryfields/circuitspecs/gaussian_unitary.py	100.00% <ø> (ø)
strawberryfields/circuitspecs/tensorflow.py	100.00% <ø> (ø)
strawberryfields/backends/fockbackend/ops.py	94.04% <94.11%> (+2.04%)	:arrow_up:
strawberryfields/ops.py	98.84% <96.29%> (-0.19%)	:arrow_down:
strawberryfields/backends/tfbackend/ops.py	96.69% <97.88%> (-0.17%)	:arrow_down:
... and 43 more

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