FFTs are in wrong direction

[darkstar007]

In the FFTW documentation here it states that:

sign is the sign of the exponent in the formula that defines the Fourier transform. It can be -1 (= FFTW_FORWARD) or +1 (= FFTW_BACKWARD). 

This comment is reflected in your file fftw.go where it says:

// sign can be -1 (= FFTW_FORWARD) or +1 (= FFTW_BACKWARD).

However your file flag.go it sets the following two constants:

// FFTW transform direction
const (
	FORWARD  = 1
	BACKWARD = -1
)

which has now swapped the directions!

Thanks,

Matt