FFT_SIZE too small for TD lowpass mode

[gfuer]

In lowpass mode, the FFT size needs to be >= 2x the number of sweep points in order that DC, Nyquist, positive frequencies and negative frequencies fit into the FFT buffer. But the definition in common.hpp does not provide that many points. The models with SWEEP_POINTS_MAX=201 would actually require 512 points, not 256.

#if SWEEP_POINTS_MAX < 256
#define FFT_SIZE 256
#elif SWEEP_POINTS_MAX < 512
#define FFT_SIZE 512
#else
#error "Need update FFT table for more points size"
#endif