FFT not working for 2D signals

[magnunor]

Pressing the FFT button for the signal s = hs.signals.Signal2D(np.random.random(size=(100,100))) gives:

Traceback (most recent call last):
  File "/home/magnunor/.virtualenvs/hyperspy_dev/lib/python3.5/site-packages/hyperspyui/plugins/fft.py", line 167, in fft
    for i in do_ffts():
  File "/home/magnunor/.virtualenvs/hyperspy_dev/lib/python3.5/site-packages/hyperspyui/plugins/fft.py", line 149, in do_ffts
    if u.endswith('-1'):
AttributeError: '_Undefined' object has no attribute 'endswith'

[magnunor]

The changes in this commit fixes this, but its quite sloppy: magnunor@7124b1e

The fs.data = np.abs(fs.data) is due to matplotlib not being able to handle imaginary data.

The if str(u.__class__) == "<class 'traits.trait_base._Undefined'>": is due to the traits.trait_base._Undefined class not having an .endswith function.