Eqsig

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* Added `response_period_range` to AccSignal object initial inputs to define response periods using an upper and lower limit
* Improved speed of surface energy calculation `calc_surface_energy` and returns correct size based on input dimensions
* Removed global import of scipy - done at function level
* Added an `interp_left` function to interpolate an array and take lower value
* Fixed issue with inverse of stockwell transform `stockwell.itransform`, it no longer doubles the time step
* Increased speed of stockwell transform `stockwell.transform`.
* Added `remove_poly` function to remove a polynomial fit from an array
* Added option to access `fa_frequencies` and `smooth_fa_frequencies` as `fa_freqs` and `smooth_fa_freqs`.
* Added option for computing smoothed FAS with extra zero padding
* Added function for computing smoothed fas using a custom smoothing matrix.

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* Added `interp2d` fast interpolation of a 2D array to obtain a new 2D array
* No longer raises warning when period is 0.0 for computing response spectrum
* Fixed issue with computation of smoothed response spectrum for dealing with zeroth frequency
* Increased speed of`generate_smooth_fa_spectrum`
* Can now directly set `AccSignal.smooth_fa_frequencies`
* Deprecated `AccSignal.smooth_freq_points` and `AccSignal.smooth_freq_range` will be removed in later version

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* More accuracy in `calc_surface_energy` - now interpolates between time steps. More tests added.

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* Fixed issue in `get_zero_crossings_array_indices` where it would fail if array did not contain any zeros.
* Added calculation of equivalent number of cycles and equivalent uniform amplitude using power law relationship as intensity measures
* Added function `get_n_cyc_array` to compute number of cycles series from a loading series
* Added intensity measure `im.calc_unit_kinetic_energy()` to compute the cumulative change in kinetic energy according to Millen et al. (2019)
* Added `surface.py` with calculation of surface energy and cumulative change in surface energy time series versus depth from surface

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* Fixed issue with second order term in sdof response spectrum calculation which effected high frequency response, updated example to show difference

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* First production release