Add functions to calculate admittance and coherence flexural properties of the lithosphere

[Khansaif1994]

I am not a professional code developer but I use built python packages for my research work.I need to compute the Admittance,coherence and other elastic properties of lithospheric plates.There is one python package for the same , called Plate flex but due to some reason its not working properly.If anyone can get the idea from Plate flex package and provide me the same it will be highly appreciable.

[santisoler]

Thanks @Khansaif1994 for opening this issue!

I think @leouieda could follow up with his ideas for such implementation here in Harmonica.

[leouieda]

@Khansaif1994 thanks for suggesting this! With the things we're already have in place for projecting grids and doing FFT analysis this should be relatively straight forward. I don't have a copy of Tony Watts' book but I know it has everything needed to do these calculations. I'll see if I can find appropriate references to check if there are any possible complications.