Add __add__ to Wires class.

[mariaschuld]

@cgogolin had the great request to add _add__ and __radd__ to the wires class as a shortcut to all_wires():

wires1 = Wires([2, 1, 3])
wires2 = Wires([1, 4, 5, 2])
wires3 = Wires([6, 5])

print(wires1 + wires2 + wires3) # <Wires = [2, 1, 3, 4, 5, 6]>

This PR adds the two functions and two tests.

Drawbacks:

Unfortunately, this does not allow native use of sum([wires1, wires2, wires3]), which would be quite elegant in our code base. sum requires a start object which is 0 by default, so the 0 wire would always be added.
One could pass the start object explicitly and use sum([wires1 + wires2 + wires3], Wires([])) which produces the same result, but is arguably more confusing than all_wires([wires1 + wires2 + wires3]).

[mariaschuld]

Snuck those in to match the correct version of the new functions.

[codecov]

Codecov Report

Merging #812 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #812   +/-   ##
=======================================
  Coverage   90.56%   90.57%           
=======================================
  Files         126      126           
  Lines        8213     8219    +6     
=======================================
+ Hits         7438     7444    +6     
  Misses        775      775           

Impacted Files	Coverage Δ
pennylane/wires.py	98.41% <100.00%> (+0.07%)	⬆️

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[josh146]

Thanks @mariaschuld! This is great. Don't forget to update the changelog as well.

By the way, I'm somewhat glad that sum([wires1, wires2, wires3]) isn't natively supported. To me, this isn't very intuitive --- on first read, I'm not sure what it would do, sum the wire labels?

I guess this is because:

• I'm so used to NumPy, where sum and concatenate are two separate operations

• sum doesn't concatenate Python lists, either (it only acts on a single list).

[josh146]

Suggested change

	**Example:**
	**Example**

😆

With Args: and Kwargs:, Sphinx now automatically removes the colon in version 2.0+ (in version 1.X, it didn't).

Also, it might be good to move it below the Args: and Returns:, to match other docstrings

[mariaschuld]

Oh yes!