For FFT example: ComplexWarning: Casting complex values to real discards the imaginary part

```
TEST >>>> test_2d_multifreq
/usr/local/lib/python2.6/dist-packages/numpy-1.6.2-py2.6-linux-x86_64.egg/numpy/core/numeric.py:235: ComplexWarning: Casting complex values to real discards the imaginary part
  return array(a, dtype, copy=False, order=order)
Several PNGs made.
```
