Add Keras layers support

[trbromley]

This PR provides a KerasLayer class for converting a PennyLane QNode to a Keras layer, which can then be used within standard Keras features such as the Model/Sequential models.

Currently, the following is not added:

• A tutorial/demo to show how to use this new feature
• A section in "Using PennyLane" of the documentation showing a higher level picture. This and the above could be added together.
• Supporting input of a QNodeCollection instead of a single QNode. This should be possible, we need to make the func.sig attribute available in the QNodeCollection and in qml.map.

Providing this functionality would help solve #166 and is also discussed in #133, https://discuss.pennylane.ai/t/qnode-inside-keras-layer/335/3 and https://discuss.pennylane.ai/t/creating-and-saving-models/46.

[codecov]

Codecov Report

Merging #529 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #529   +/-   ##
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  Coverage   98.89%   98.89%           
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  Files          82       82           
  Lines        5075     5075           
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  Hits         5019     5019           
  Misses         56       56           

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[trbromley]

Thanks @josh146! Should have answered your questions.

[josh146]

💯

[co9olguy]

🎆