Disallow taking the adjoint of fractional power operations

[dwierichs]

Context:
The adjoint of an integer power of an operator is the same power of the adjoint of the operator.
For fractional powers, this does not hold because of branch cuts in the power function.

Description of the Change:
This PR explicitly disallows computing the (eager) adjoint of a Pow operator with fractional power, and raises an AdjointUndefinedError in this case.

As a tiny side effect, this PR changes the signature of Identity.pow to be compatible with generalized simplification workflows where the keyword argument z for the power is passed around explicitly (this popped up in a simplification test of Pow)

NB: As usual, a lazy Adjoint(Pow(base, z=0.2)) is still supported, just can't be evaluated/simplified if that would lead to calling the method adjoint of Pow.

Benefits:

Possible Drawbacks:

Related GitHub Issues:
Fixes #5812

[sc-65297]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (45f8d4a) to head (cd09001).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5835      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         420      420              
  Lines       40138    39847     -291     
==========================================
- Hits        40010    39717     -293     
- Misses        128      130       +2     

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[EmilianoG-byte]

This PR looks good! All my comments are related to the case of whole-floats (e.g. z = 2.0) which in my opinion should be treated as integers. But of course, I am open for discussion 😄

[albi3ro]

We can handle exponents like z=1.0 at some later point. This fixes the issue.

[EmilianoG-byte]

Based on our discussion, I also agree the current changes are fine for now 😃. Thank you for the discussion!