[pymvpa] Time for single permutation varies between a few seconds and several minutes

[Jan Derrfuss]

Some additional observations:

- I ran 100 searchlights using the same dataset to see if the issue also 
occurs; it does not, all searchlights took 11-12 seconds

- when the permutation takes minutes, the times are not random; they 
were (in s): 225, 236, 225, 611, 241, 247, 267, 232; for the times in 
the 200's, this is roughly the time for one fast permutation multiplied 
by 20

- the effect also occurs with assure=False

Jan