Add STFT & Synshrosqueezed STFT

[OverLordGoldDragon]

FEATURES:

• stft, istft, ssq_stft, and issq_stft implemented and validated
• Added to utils.py: buffer, unbuffer, window_norm, window_resolution, and window_area
• Replaced numba.njit with numba.jit(nopython=True, cache=True), accelerating recomputing

BREAKING:

• cwt() no longer returns x_mean
• padsignal now only returns padded input by default; get_params=True for old behavior
• Moved methods: phase_cwt & phase_cwt_num from ssqueezing to _ssq_cwt
• In future release: return order of cwt and stft will be changed to have Wx, dWx and Sx, dSx, and ssq_cwt and ssq_stft to have Tx, Wx and Tx, Sx

MISC:

• wavelet positional argument in cwt is now a keyword argument that defaults to 'morlet'
• Support for padsignal(padtype='wrap')
• Docstring, comment cleanups

[OverLordGoldDragon]

I see this paper's possibly main rationale (though unstated) for ssqueezing according to a Lebesgue measure (ones(Sx.shape)): stability. The STFT implementation takes FFTs along columns rather than rows; this creates extreme sensitivity to imperfections in the phase transform: for a given column, values in rows plot into wavelet-like oscillations, meaning upon summation (ssqueezing), exclusion of even a single value greatly impacts final value in a given Tx location.

I've verified this with a perfectly set up pure sinusoid (no shadow frequencies); perhaps the remedy is in also using a perfect window, but this many perfections are hardly realistic (or generalizable). Since Lebesgue sets all values in Sx to 1, exclusion of any one value is no big deal, and we see a dramatic improvement:

lebesgue's main strength is also its main weakness: values away from ridge (STFT max row) are (as should be) much smaller, but are counted as equals, which exacerbates noise energy and general perturbations. A simple compromise, then, is to sum abs(Sx) - which seems to work ideally:

Problem is, we may lose invertibility, but not necessarily by a lot. I've yet to gauge effectiveness for other kinds of signals.

[OverLordGoldDragon]

'Modified' STFT solves the problem quite neatly.