Deprecate (faulty, according to Stephan Preibisch) legacy FFT

The phase correlation is still missing, but the better way to do FFT
with ImgLib2 is in imglib2.algorithm.fft2.* and should be used instead.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge pull request #36 from imagej/deprecate-legacy-fft

Deprecate (faulty, according to Stephan Preibisch) legacy FFT