Use comma when printing the wires of qml.Hermitian in qml.Hamiltonian

[antalszava]

Context

When printing the observables building up qml.Hamiltonian, coefficients and terms are printed following the OpenFermion convention. OpenFermion doesn't represent aribtrary Hermitian observables, therefore we're using a custom notation for qml.Hermitian (#987):

  (0.8) [Z0]
+ (0.8) [Z0 Hermitian1'3 Z2]

Changes
Changes the separator between the wires of qml.Hermitian to be a comma:

  (0.8) [Z0]
+ (0.8) [Z0 Hermitian1,3 Z2]

Related issues
Closes #1068

[github-actions]

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[codecov]

Codecov Report

Merging #1465 (f6f96d0) into master (148d9cc) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #1465   +/-   ##
=======================================
  Coverage   98.29%   98.29%           
=======================================
  Files         169      169           
  Lines       12360    12360           
=======================================
  Hits        12149    12149           
  Misses        211      211           

Impacted Files	Coverage Δ
pennylane/vqe/vqe.py	92.19% <0.00%> (ø)

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[albi3ro]

Seem like more people want the commas. Thanks for fixing this up.