Add discrete fourier transform as an AutoEumlateTransform

[sgreenbury]

Contributes towards #513.

This PR adds:

• DiscreteFourierTransform subclassing AutoEmulateTransform
• Example usage in exploratory notebook comparing with PCA

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Codecov Report

❌ Patch coverage is 96.32353% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.39%. Comparing base (cecf95d) to head (d4456c7).

Files with missing lines	Patch %	Lines
...mulate/experimental/transforms/discrete_fourier.py	93.75%	3 Missing ⚠️
autoemulate/experimental/compare.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #655      +/-   ##
==========================================
+ Coverage   80.20%   80.39%   +0.19%     
==========================================
  Files         155      157       +2     
  Lines       11132    11259     +127     
==========================================
+ Hits         8928     9052     +124     
- Misses       2204     2207       +3     

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
autoemulate/experimental
compare.py					294-299
autoemulate/experimental/transforms
__init__.py
discrete_fourier.py					157-162
tests
test_compare.py
tests/experimental/transforms
test_discrete_fourier.py
test_serde.py
test_transforms.py
Project Total

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