[WIP] Add is_hermitian property to Operator

[Jaybsoni]

Context:
Typically we use the type of an object to determine whether it can be used in a MeasurementProcess. We explicitly check if the type is Observable. This conflicts with operator-arithmetic classes which may or may not be observables depending on the instance. (eg. a Sum of observables may not be an observable unless their coefficients were real)

Description of the Change:
Add a property is_hermitian to the base Operator class, use this to determine if an operator can be used in a MeasurementProcess.

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

Merging #2629 (3bf72ba) into master (e40c47e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2629   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files         244      244           
  Lines       19667    19673    +6     
=======================================
+ Hits        19585    19591    +6     
  Misses         82       82           

Impacted Files	Coverage Δ
pennylane/measurements.py	99.57% <100.00%> (ø)
pennylane/operation.py	96.54% <100.00%> (+0.03%)	⬆️

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[Jaybsoni]

[sc-20262]

[albi3ro]

Looks good 👍