performance optimize _generate_fourier_series_np

[mathause]

• Closes optimize _generate_fourier_series_np #410
• Tests added
• Fully documented, including CHANGELOG.rst

Performance optimization of _generate_fourier_series_np. Builds on #512 so the diff is actually smaller. This does two main things:

1. cos(alpha) is periodic - so we only need to compute it once for each month (instead of n_years * 12 times). We then reshape the yearly_predictor to n_years x 12 such that broadcasting ensures each element is multiplied with the coefficients
2. Change the order of operation: sum the coefficients before multiplying - this avoids a lot of multiplication and sum operations. So instead of
   $$\sum\left((a_i \cdot T + b_i) \cdot \cos(\alpha) + (c_i \cdot T + d_i) \cdot \sin(\alpha)\right)$$
   we do
   $$\sum\left(a_i \cdot \cos(\alpha) + c_i \cdot \sin(\alpha)\right) \cdot T + \sum\left(b_i \cdot \cos(\alpha) + d_i \cdot \sin(\alpha)\right)$$

This speeds up _generate_fourier_series_np about 9 times and fit_harmonic_model about 4 times (on my machine). We do pay this with a small loss in precision (see changes in tests)

---

What we do additionally:

1. Combine the $\cos$ and $\sin$ array into one so we can directly multiply them with the coeffs
2. use @ to compute the product-sum
3. optimize the creation of the alpha array for the cost of some readability

What else could we do:

1. Split the function into two to pre-allocate the cos_sin array similar yeo-johnson: performance optimization #496
2. use fft to get better inital conditions (fourier series: more consistent with standard definition #512 (comment))

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 49.85%. Comparing base (444c682) to head (75b6f70).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #516      +/-   ##
==========================================
+ Coverage   49.73%   49.85%   +0.12%     
==========================================
  Files          50       50              
  Lines        3557     3566       +9     
==========================================
+ Hits         1769     1778       +9     
  Misses       1788     1788              

Flag	Coverage Δ
unittests	49.85% <100.00%> (+0.12%)	⬆️

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[mathause]

This suggestion may be relevant for https://github.com/MESMER-group/mesmer/pull/501/files#r1738976265

cc @veni-vidi-vici-dormivi

[veni-vidi-vici-dormivi]

Suggested change

	- Refactor variable names, small code improvements, fixes and clean docstring
	- Refactor variable names, small code improvements, optimization, fixes and clean docstring

[mathause]

Thanks for the review. Let's merge.