Add tests for exponential extrapolation

[natestemen]

Context: The last PR (first, second) to add exponential extrapolation capabilities to catalyst.

Description of the Change: This PR ensures that pennylane.transforms.exponential_extrapolate works inside catalyst's mitigate_with_zne function.

Benefits: ZNE has increased functionality.

Possible Drawbacks: As part of the testing, I've removed 0.1 and 0.2 from the list of values being used to create different circuits. This is because exponential extrapolation is not necessarily as stable as polynomial fitting. For example, when extrapolating near constant values exponential extrapolation can struggle due to the fact that a linear fit is first performed to understand the "direction" (or sign) of the exponential. If the slope is positive, the data is flipped to fit something that looks like $A\mathrm{e}^{-Bx} + C$ where $B\in\mathbb{R}_{> 0}$.

An example of this happening can be seen here.

>>> exponential_extrapolate([1,2,3], [0.3894, 0.3894183, 0.38941])
-1.0000000000000059e-06
>>> richardson_extrapolate([1,2,3], [0.3894, 0.3894183, 0.38941])
0.3893551000000072

If we want to ensure to continue testing values 0.1 and 0.2 for polynomial extrapolation, we can create separate tests. Let me know what would be best.

Related GitHub Issues:

fixes #754

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.93%. Comparing base (4f2b9f1) to head (127116f).
Report is 224 commits behind head on main.

Additional details and impacted files

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  Files          73       73           
  Lines       10338    10338           
  Branches     1170     1170           
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  Hits        10125    10125           
  Misses        170      170           
  Partials       43       43           

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[natestemen]

just pinging @rmoyard / @dime10 / @josh146 in case y'all didn't see the discord message.

[cosenal]

Lgtm

[rmoyard]

@natestemen could keep all the values but add a skip in the test directly for those values only for exponential? With the skip add a comment about exponential not being stable.

[rmoyard]

Closes #754

[rmoyard]

Thanks 💯

[natestemen]

@rmoyard just added a snippet to the changelog, and added a missing docstring to satisfy CI. Feel free to take another look, or merge!