Deprecating dft and replacing it with fft

[roxyboy]

A PR to initiate a deprecation cycle for xrft.dft and to replace it with xrft.fft. As the tests all pass using dft, none of the previous behaviors should break with only the warning messages changing.

[roxyboy]

@santisoler @lanougue Do you have any input on this? Follow our discussion in #154, I tried unifying xrft to fft.

[santisoler]

Looks good to me! 🙌🏼

[roxyboy]

@rabernat This deprecation cycle of removing dft and unifying the default function to fft will result in quite a drastic change down the line but would you agree with this? It seems that having two functions (fft and dft) is confusing to users.