Allow wires to be passed as final positional argument

[albi3ro]

Currently, the initialization of Observable has logic that allows the wires to be passed as the final positional argument.

I move this logic to Operator to make it more general. This also allows us to get rid of a specific __init__ function for Observable's.

I also tidied two other chunks in the Operator initialization function while I was there.

[codecov]

Codecov Report

Merging #2432 (979aeef) into master (c272db0) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2432      +/-   ##
==========================================
- Coverage   99.47%   99.47%   -0.01%     
==========================================
  Files         245      245              
  Lines       19495    19493       -2     
==========================================
- Hits        19393    19391       -2     
  Misses        102      102              

Impacted Files	Coverage Δ
pennylane/operation.py	96.63% <100.00%> (-0.02%)	⬇️

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[Jaybsoni]

Looks pretty straight forward 👍🏼

[antalszava]

The Codecov failure seems to be a false negative, overriding the check.