pyfftw release references to arrays without destroying plan

FFTW plans are by there nature tied to a piece of memory. However, there is nothing to stop you using the same piece of memory for all your plans. So you could create a single array that is big enough for all your possible arrays and then create your FFTW objects on views into that array.

You can then execute the FFT using the FFTW.__call__() interface that allows the arrays to be updated prior to execution (with little overhead when they agree with the original array in strides and alignment).

Now, the FFTW object will have the new arrays as its internal arrays. If you want to revert back to the other memory, you can use FFTW.update_arrays().