Assert spec and layout target equality

[thisac]

Context: Currently, there's no check against using different targets in the specification (Device.target, or Device._spec["target"]) and in the Blackbird layout (target field in Device.layout, or BlackbirdProgram.target). This shouldn't be an issue for the user unless the specification is incorrect, although it's safer to have a check for this at device creation directly.

Description of the Change:

• A Device.assert_spec static method is added which checks that the target in the layout is the same as the target field in the specification. This check is also performed at Device initialization.
• If no target is specified in the layout, the device target is used instead. This is useful e.g., when compiling a program with a device (currently, if this would be attempted without a target in the layout, an obscure CircuitError would be raised.

Benefits:

• The target in the layout can either be omitted, or must be the same as the target in the specification.

Possible Drawbacks: None

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

Merging #687 (8b6d019) into master (627b491) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #687   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files          75       75           
  Lines        9198     9209   +11     
=======================================
+ Hits         9078     9089   +11     
  Misses        120      120           

Impacted Files	Coverage Δ
strawberryfields/device.py	97.18% <100.00%> (+0.40%)	:arrow_up:
strawberryfields/program.py	100.00% <100.00%> (ø)

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