support iswap, siswap, and their adjoints

[timmysilv]

As the title says. These gates have native counterparts in cirq. I will work on making general adjoint support for plugins once all the native gates needing work are supported right out of the gate (hehehe)

[codecov]

Codecov Report

Merging #114 (81e7560) into master (fe2c417) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #114   +/-   ##
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  Coverage   99.23%   99.23%           
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  Files           8        8           
  Lines         393      393           
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  Hits          390      390           
  Misses          3        3           

Impacted Files	Coverage Δ
pennylane_cirq/cirq_device.py	98.92% <ø> (ø)

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[albi3ro]

Looks good 👍

I wonder if we could define the Adjoint(S) and Adjoint(T) as instances of the cirq.ZPowGate? But that's something we can further look into later.

[timmysilv]

I too think that's the way! I'm planning on using those PowGate classes to implement a generic solution for adjoints and powers after this PR