Infer harmonic model order from coefficients

[veni-vidi-vici-dormivi]

In the harmonic model generate_fourier_series we passed the order as an argument instead of inferring it from the length of the coefficients array. This is changed here. Additionally, I reordered the arguments to facilitate testing on xarrays.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 81.94%. Comparing base (9b0b76b) to head (7094488).
Report is 30 commits behind head on main.

Files	Patch %	Lines
mesmer/mesmer_m/harmonic_model.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #434      +/-   ##
==========================================
- Coverage   87.90%   81.94%   -5.96%     
==========================================
  Files          40       44       +4     
  Lines        1745     1939     +194     
==========================================
+ Hits         1534     1589      +55     
- Misses        211      350     +139     

Flag	Coverage Δ
unittests	81.94% <50.00%> (-5.96%)	⬇️

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[mathause]

I think it's a good idea to rename the function and make it private but that can be another PR (#410).