support for ops raised to a power; remove old custom ops

[timmysilv]

Introduce support for operations raised to a power using Cirq PowGates.

List of Cirq PowGates:

➜  ops pwd                                       
/Users/matthews/.pyenv/versions/3.9.13/envs/pl_plugins/lib/python3.9/site-packages/cirq_core-1.1.0.dev20220917023638-py3.9.egg/cirq/ops
➜  ops grep -ohrE "class \w+PowGate" .                    
class SwapPowGate
class ISwapPowGate
class CustomXPowGate
class XPowGate
class YPowGate
class ZPowGate
class HPowGate
class CZPowGate
class CXPowGate
class XXPowGate
class YYPowGate
class ZZPowGate
class CCZPowGate
class CCXPowGate
class PhasedISwapPowGate
class PhasedXPowGate
class WeightedZPowGate

As well, pennylane_cirq supports gates that exist in Cirq but not in PennyLane. This included CPhase and ISWAP, but those exist in PennyLane! I've removed the non-PL operation support for those and ensured the PL operation support.

[codecov]

Codecov Report

Merging #115 (0adaad7) into master (519d027) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
- Coverage   99.23%   99.23%   -0.01%     
==========================================
  Files           8        8              
  Lines         393      392       -1     
==========================================
- Hits          390      389       -1     
  Misses          3        3              

Impacted Files	Coverage Δ
pennylane_cirq/ops.py	100.00% <ø> (ø)
pennylane_cirq/__init__.py	100.00% <100.00%> (ø)
pennylane_cirq/cirq_device.py	99.03% <100.00%> (+0.11%)	⬆️

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[AlbertMitjans]

Good job! Left some comments

[albi3ro]

Should stick this under "breaking changes".

[albi3ro]

Nice solution! Thanks for this PR :)

[AlbertMitjans]

Good job!