[FEATURE] add extrapolate, IIR extrapolation of a spectrum beyond its range - #121

Adds extrapolate(signal, freqRange, order=4) to pytta.functions, the extrapolation of a spectrum beyond its valid range of ita_extrapolate of the ITA-Toolbox. Outside freqRange = [f1, f2] the spectrum is replaced by IIR models fitted near each end with invfreqz (#110). The low model is fitted on f1 + (0.01 to 0.2)(f2 - f1) and used below f1, and the high model on f1 + (0.9 to 0.99)(f2 - f1) and used above f2, the bands of the ITA-Toolbox.

The ITA-Toolbox fits with MATLAB's rationalfit (RF Toolbox), which has no SciPy counterpart; the equation error fit of invfreqz takes its place. A pure delay is not a low-order model, so it should be removed first (time_shift of #96); the ITA-Toolbox removes it inside the function. The ITA code was read for its semantics only and nothing was copied.

This pull request builds on #110 and shows its commit until #110 is merged.

The test in tests/test_extrapolate.py takes a band-pass filter of order 4, replaces its spectrum outside 300 Hz to 3 kHz by noise, and extrapolates with models of order 4: the spectrum comes back outside that range within 3.2e-4 of its peak, which is what the float32 samples of SignalObj allow.

Checked with Python 3.12, numpy 1.26 and scipy 1.17. The 7 failures of development (3 in test_h5io.py, 4 in test_pyttasave.py) are the same with and without this change.

Co-Authored-By: Claude Fable 5.1 noreply@anthropic.com