PennyLane v0.4 support

[josh146]

Updating the plugin to work with PL version 0.4. Changes include:

• Adds support for PennyLane version 0.4. This includes:

  • Renaming expectation -> observable in various places in the documentation
  • Device.expectations -> Device.observables
  • Device.pre_expval -> Device.pre_measure
  • Device.post_expval -> Device.post_measure

• Adds the device method var() to all devices

• Adds corresponding tests, to ensure the variance is correctly calculated.

• Updates the minimum required PennyLane version to 0.4

• Update the documentation to reflect PennyLane v0.4 support

• Increment plugin version number to 0.4.0

Once this PR is merged, I will update the PyPI release.

[codecov[bot]]

Codecov Report

Merging #51 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #51   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines         217    229   +12     
=====================================
+ Hits          217    229   +12

Impacted Files	Coverage Δ
pennylane_pq/_version.py	100% <100%> (ø)	:arrow_up:
pennylane_pq/devices.py	100% <100%> (ø)	:arrow_up:

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