This module is no longer required for the 1D Fast Fourier transform 
({\sf fft1d} module), but can be useful to speed-up computations, though
the more efficient module {\sf fft1dshrink} should be prefered in general.
It extracts of an input signal the largest centered part whose dimension 
is a power of $2$.
\index{crop!a signal}
For example, an original signal of size $600$ will be shrinked to 
size $512$, by extracting the part $[44,555]$.

\vskip 0.5cm

NB: Calling this C-subroutine with $out=in$ is nonsense.
