Verify against double-FFTw, benchmark against float-FFTw
Add builtin geometry functions
Add cl_convert<T>() as convert_T() equivalent
Construct vector from existing cl::Buffer
Use Bluestein Chirp Z algorithm.
If a 1D-FFT size can't be factorized into existing kernels, use Bluestein for the whole FFT.
Very large error for single precision.
Next step will be adding this into the Cooley-Tukey recursion so a size like 1024x1009 will only have to use Bluestein for n=1009, not (as is currently the case) n=1033216, which will increase accuracy.
Verify float/double CLFFT against double FFTw
Merge remote-tracking branch 'neapel/master'

Conflicts:
	examples/fft_verify.cpp
	vexcl/fft/kernels.hpp