Move is_commuting to pennylane/ops/functions

[albi3ro]

I need to figure out how is_commuting works to extend it for arithmetic ops like Controlled.

While it was created in the development of the CommutationDAG and used by the commutation dag, pennylane/transforms/commutation_dag.py is a fairly non-intuitive place for a top-level function is_commuting.

As it is a function of operators, this PR moves it to where we place our functions of operators.

No changes are made to the source code, though we should try and improve how this function works.

[codecov]

Codecov Report

Merging #2991 (d00f7bf) into master (352947b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2991   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files         266      267    +1     
  Lines       22397    22400    +3     
=======================================
+ Hits        22320    22323    +3     
  Misses         77       77           

Impacted Files	Coverage Δ
pennylane/__init__.py	100.00% <ø> (ø)
pennylane/transforms/__init__.py	100.00% <ø> (ø)
pennylane/ops/functions/__init__.py	100.00% <100.00%> (ø)
pennylane/ops/functions/is_commuting.py	100.00% <100.00%> (ø)
pennylane/transforms/commutation_dag.py	100.00% <100.00%> (ø)

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[Jaybsoni]

Looks good to me !
Just one comment, in the documentation we have a subheading Quantum Operators where we list operator functions like is_hermitian and is_unitary. It might be worth adding this to that table

[Jaybsoni]

Ready to go 👍🏼