Add a fallback for FFT when VkFFT is not found or broken (#130)

* use loopy fft

* fix inverse

* Implement broadcasting FFT

* use enum for fft backend

* Add gh-129 link

* unit test for loopy_fft

* Unit test for loopy_fft and fix warnings

* don't use vkfft only if x86 mac

* Add missing import

* Fix platform.machine()