Merge the demo branch into master

[josh146]

This PR merges the demo branch into master in preparation for release. In particular, it adds the following features which so far have been developed in isolation in the demo branch:

• The Connection and Job classes

• The RemoteEngine

• The sf command line interface

• The sf.configuration module

• Documentation and tests related to the above, including a tutorial tutorials/remote.rst walking through usage of RemoteEngine.

While this PR is large, it is composed of code-reviewed PRs, it is still worth giving it a quick once-over.

Note: please have a look at the updated changelog (rendered version here: https://strawberryfields.readthedocs.io/en/demo/development/release_notes.html). I have collated several independent PRs into single bullet points, since this will be squash-merged into master, and added examples.

• @nquesada and @thisac, do you have a nice plot we could add to the 'speeding up the fock backend' change?

• @trbromley, do you have a nice small example we could add to the 'weighted graphs' change?

[codecov[bot]]

Codecov Report

Merging #319 into master will not change coverage by %. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #319   +/-   ##
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  Coverage   97.66%   97.66%           
=======================================
  Files          50       50           
  Lines        6252     6252           
=======================================
  Hits         6106     6106           
  Misses        146      146           

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[josh146]

@trbromley, thanks! I agree with your comments re: the configuration page and the hardware tutorial. However, I think it's best to address them in separate PRs post-demo merge. Partly because this PR is so big, and blocking a few other changes required for release.

[trbromley]

@trbromley, do you have a nice small example we could add to the 'weighted graphs' change?

This is addressed in https://github.com/XanaduAI/strawberryfields/pull/321 - should be a few minutes to review, but wanted to do as a PR to keep the change clear.