feat: improve stft plot computation time

[ansaminard]

Plotting STFTs would be quite time-consuming, for 2 reasons:

• conversion of STFT data from FieldsContainer to numpy array was inefficient
• when plotting an STFT, this conversion was done twice, when once only would have sufficed

To remedy these:

• conversion now preallocates array memory before looping on the FieldsContainer's fields
• conversion called once only when plotting

More generally, these changes improve computation times in any situation where the STFT data is being retrieved as a numpy array (meaning it also improves computation times for some other example scripts that this PR did not change)

Computation times during CI/CD before these changes:

Computation times during CI/CD after these changes:

(also improved some minor code readability issues)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (28eceb1) to head (dbcf17f).
Report is 1 commits behind head on main.

Additional details and impacted files

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##              main      #287   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           81        81           
  Lines         4977      4976    -1     
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- Hits          4977      4976    -1     

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