Remove power transformer class

[veni-vidi-vici-dormivi]

• Closes test PowerTransformer against sklearn (for constant lambda) #462
• Tests added
• Fully documented, including CHANGELOG.rst

In this PR I am removing the power transformer class in favor of the newly written xarray functions and adjusting the tests accordingly.

[veni-vidi-vici-dormivi]

I now deleted the PowerTransformerVariableLambda class. Should we instead deprecate it properly? I thought it is unnecessary because because it was never properly implemented. What do you think @mathause?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.53%. Comparing base (9b0b76b) to head (4745f11).
Report is 104 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #474       +/-   ##
===========================================
- Coverage   87.90%   51.53%   -36.38%     
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  Files          40       50       +10     
  Lines        1745     3252     +1507     
===========================================
+ Hits         1534     1676      +142     
- Misses        211     1576     +1365     

Flag	Coverage Δ
unittests	51.53% <ø> (-36.38%)	⬇️

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[mathause]

Good for me to just remove it. It also disentangles it as the PoerTransformer can also do a box-cox transformation which was however not adapted for variable $\lambda$.

[veni-vidi-vici-dormivi]

I think the only point where we still need sklearn now is the tests. So do I see that correctly that we could now (again) have a separate environment for testing and for the users and exclude sklearn in the users environment, or not? Even if so, I would keep it at one environment at this point I think.

[mathause]

It's always nice to have less dependencies but we can keep it for the moment. Maybe add a comment in setup.cfg?