Speed up function using fftw
Speed up using fftw and different lowpass mask

Instead of moving the FFT'ed data, then apply a mask, then shift the
image back again we pre-shift the lowpass mask and use that instead.
Bugfix: need subaperture size (sasz) in sim_shwfs now
Disabled pyfftw due to potential caching memleak

PyFFTW might have a memory leak when using caching, and without caching
its too slow for this specific application, so I disabled it.
Use long caching timeout, copy returned array

- Inrease PyFFTW caching timeout to prevent race condition
- Copy returned array to prevent potential memory leak(?)