BF+DOC: fixing permutation test example
BF+DOC: fixing permutation test example

Removing redundant code.
Remove assumption that images can be treated as array-like.
Add explanatory docstring

arokem/nipy,arokem/nipy,alexis-roche/register,bthirion/nipy,bthirion/nipy,nipy/nipy-labs,alexis-roche/nipy,alexis-roche/nireg,alexis-roche/nipy,alexis-roche/nipy,bthirion/nipy,alexis-roche/register,alexis-roche/niseg,alexis-roche/nipy,nipy/nireg,alexis-roche/niseg,bthirion/nipy,nipy/nireg,alexis-roche/nireg,alexis-roche/register,nipy/nipy-labs,arokem/nipy,arokem/nipy