Interface module for FFT methods

[pysteps]

Interface module for FFT methods

[aperezhortal]

@pulkkins , since the fft method is used in several modules, another alternative to the interface is implementing something similar the matplotliob.use function used to set the plot backends.

The default fft method can be defined in the rcparams by default, and changed by the the user.
Then, fft method can be changed in runtime by calling a function pysteps.set_fft("fft_method").

To force all the modules to use the selected fft, the pysteps modules can make use of a pysteps.fft function that is updated by the set_fft method changing its reference to the corresponding method. When the user select a library that is not installed, an exception is raised.

In this way, the sources that use the fft only need to import it as **from pysteps import fft".

[dnerini]

I agree with @aperezhortal, we could actually use the rcparms for many of the parameters in pysteps, thus avoiding having every time to pass a long list of arguments when calling a nowcasting method.

[pulkkins]

The FFT interface module has been implemented. I'll leave this issue open because using rcparams for choosing the FFT method still needs to be implemented.