Support fp16 and bf16 precision

[zhuohoudeputao]

Since increasingly deep learning or ai related computation relies on FFT and calculates half precision, is there any plan to support fp16 and bf16 precision for fftw?

[stevengj]

It should be straightforward to add, in principle. The scalar case would be trivial to add, while SIMD support will take a bit more work.

[zhuohoudeputao]

libbench should also support testing for half precision, too.