copy ricker wavelet from scipy which removed it from version 1.15.0

[wenh06]

This PR fixes issue #525

[tompollard]

Thanks @wenh06! If copying, we need to make sure that we follow the license terms, which I think are here: https://github.com/scipy/scipy/blob/main/LICENSE.txt

Alternatively, we could add a dependency on the recommended package: https://pywavelets.readthedocs.io/en/latest/

[wenh06]

The ricker wavelet is defined as

$$ \psi (t)={\frac {2}{{\sqrt {3\sigma }}\pi ^{1/4}}}\left(1-\left({\frac {t}{\sigma }}\right)^{2}\right)e^{-{\frac {1}{2} \left({\frac {t}{\sigma }}\right)^{2}}} $$

But pywt seems to fix $\sigma = 1$, I don't know how to change this parameter with pywt.

[tompollard]

Looks good to me, thanks again @wenh06