Fixed point works (in the loosest sense of the word "works")
Fixed point works (in the loosest sense of the word "works")

Fixed point sums are divided by 2 each stage.  This will never overflow for radix 2 ffts.
For mixed radix, it may overflow, but will usually give better SNR.

'make test' output:

### testing SNR for  1024 point FFTs
#### DOUBLE
snr_t2f = 295.30
snr_f2t = 308.25
#### FLOAT
snr_t2f = 146.92
snr_f2t = 143.25
#### SHORT
snr_t2f = 54.645
snr_f2t = 24.677

#### timing 10000 x 1024 point FFTs
#### DOUBLE
Elapsed:0:25.96 user:19.77 sys:0.22
#### FLOAT
Elapsed:0:06.62 user:5.48 sys:0.11
#### SHORT
Elapsed:0:06.01 user:4.75 sys:0.12

sf-is-evil/kissfft,sf-is-evil/kissfft,pemryan/kissFFT,pemryan/kissFFT,pemryan/kissFFT,sf-is-evil/kissfft,sf-is-evil/kissfft,pemryan/kissFFT