Implement Generalized Morse Wavelet Transform

Hi everyone,

After discussions within the team (@jcmosher, @yashvakilna et al.), the team recommends that we update Brainstorm’s wavelet‐transform module and add the generalized Morse wavelet as the default option. This change will bring several advantages. 

https://www.mathworks.com/help/wavelet/ug/morse-wavelets.html

Cheers