Add canonical 2-complete linear swap network

[obliviateandsurrender]

Context: Adds 2-CCL swap network described in arXiv:1905.05118

Description of the Change: Adds a new template category for the swap network

Benefits:

Possible Drawbacks: N/A

Related GitHub Issues:: N/A

[codecov]

Codecov Report

Merging #3447 (29f160b) into master (e2e23e5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3447   +/-   ##
=======================================
  Coverage   99.87%   99.87%           
=======================================
  Files         311      313    +2     
  Lines       27933    27984   +51     
=======================================
+ Hits        27897    27948   +51     
  Misses         36       36           

Impacted Files	Coverage Δ
pennylane/__init__.py	96.73% <100.00%> (+0.03%)	⬆️
pennylane/templates/__init__.py	100.00% <100.00%> (ø)
pennylane/templates/swapnetworks/__init__.py	100.00% <100.00%> (ø)
pennylane/templates/swapnetworks/ccl2.py	100.00% <100.00%> (ø)

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[obliviateandsurrender]

I will remove the FermionicSWAP code from here once the previous PR gets merged.

[DSGuala]

Looking good so far! I left a lot of comments and suggestions and I'm requesting changes. But I think they shouldn't be too hard to address. They're smaller problems.

[DSGuala]

Left few more comments, but I approve as soon as those are addressed. Everything works really well, it just takes some time to read through and understand how to use the SWAP network to do specific things.