ffts

Merge pull request #58 from linkotec/master

Add support for non-power of 2 FFT sizes using Chirp z-transform a.k.a Bluestein algorithm

Rename double_t to ffts_double_t, and use for all trig tables

New functions ffts_cexp_32f to calculate cos(pi * n / d) and sin(pi * n / d), ffts_generate_chirp_32f to generate chirp z-transform sequence, and replace old ffts_generate_cosine_sine_32f with more accurate algorithm.

Improve const correctness