[unitaryHACK] Added SWAP gate decomposition and related tests

[charmerDark]

Context:
Added SWAP decomposition and wrote tests to it

Description of the Change:
added the the function

 @staticmethod
   def decomposition(wires): 

to pennylane/ops/qubit.py and tests for it at tests/ops/test_qubit_ops.py

Possible Drawbacks:
similar to #1313
Related GitHub Issues:
resolves #1304

[codecov]

Codecov Report

Merging #1329 (23a3456) into master (2107490) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1329   +/-   ##
=======================================
  Coverage   98.16%   98.16%           
=======================================
  Files         148      148           
  Lines       11446    11450    +4     
=======================================
+ Hits        11236    11240    +4     
  Misses        210      210           

Impacted Files	Coverage Δ
pennylane/ops/qubit.py	98.50% <100.00%> (+<0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2107490...23a3456. Read the comment docs.

[antalszava]

Hi @charmerDark, this looks good to me! 🎉 🙂 Extra credit for going through the challenges with git. Thank you for your contribution! :)

[charmerDark]

Thank you so much @antalszava Couldn't have made it without all your support. I learnt so much from this. Hope to be able to contribute more to pennylane soon.