Code quality and API improvements

[raphaelvallat]

Bug fixes

• epsilon value corrected (utils.py): epsilon = 10e-9 (= 1e-8) was a silent typo; fixed to 1e-9. This constant guards against division by zero in the Numba linear regression used by detrended_fluctuation.
• __all__ instead of all (entropy.py, fractal.py, utils.py): All three modules assigned to a bare all variable, inadvertently exporting every imported name (np, jit, KDTree, …) on from antropy import *. Fixed
  to __all__
• sample_entropy undefined case returns np.nan: When no template of length m matches within the tolerance (denominator = 0), the function previously returned 0. This is mathematically undefined.

API additions

• hjorth_params(sf=None): New optional sf parameter converts mobility from samples⁻¹ to Hz by multiplying by the sampling frequency. Complexity is unaffected (dimensionless ratio).

Documentation

• lziv_complexity: Added .. warning:: about float/int arrays being truncated to uint32
• hjorth_params: Added note clarifying mobility units and sf conversion; updated example.
• num_zerocross: Added Notes section documenting signbit(0) == False behaviour (a transition through exactly 0 counts as one crossing, not two).
• spectral_entropy: Added note that the DC component (f = 0) is included in the entropy sum.
• detrended_fluctuation: Added note that α ≈ 1 is ambiguous between the fGn/fBm regimes.
• svd_entropy: Clarified normalisation wording (maximum is reached when all singular values are equal).
• Removed stale Python 2 comment (np.true_divide) and dead Timmermann et al. commented-out code block.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (372dd11) to head (183f641).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #46   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          302       312   +10     
  Branches        45        52    +7     
=========================================
+ Hits           302       312   +10     

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