Define returns_state in capabilities

[antalszava]

The Cirq devices that define the state method are capable of returning the state, yet they don't have the returns_state=True entry in the capabilities dictionary of the device (see the explanation of capabilities here).

[codecov]

Codecov Report

Merging #107 (61583e8) into master (ad95aad) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   99.23%   99.24%   +0.01%     
==========================================
  Files           8        8              
  Lines         391      397       +6     
==========================================
+ Hits          388      394       +6     
  Misses          3        3              

Impacted Files	Coverage Δ
pennylane_cirq/qsim_device.py	96.87% <100.00%> (-0.10%)	⬇️
pennylane_cirq/simulator_device.py	100.00% <100.00%> (ø)

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[rmoyard]

Thank you @antalszava, it looks good to me 💯