Eqsig

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* Added tol threshold for zero crossing and peak indices algorithms

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* Adjusted `eqsig.stockwell.plot_stock`, since min freq was out by factor of 0.5.

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* Added `gen_ricker_wavelet_asig` to create an acceleration signal that is a Ricker wavelet
* Added `eqsig.sdof.calc_input_energy_spectrum` to compute the input energy into an SDOF
* Can now load a Signal with a scale factor by passing in the keyword `m=<scale factor>`
* The left interpolation function interp_left now returns the same size as x, which can be a scalar, and if `y` is None then assumes index (0,1,2,...,n)

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* Fixed issue with computation of surface energy spectra
* Support for numpy==1.19

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* Fixed docs for generation of FAS, changed kwarg `n_plus` to `p2_plus` since this adds to the power of 2.

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* Switched to numpy for computing the Fourier amplitude spectrum