peak_from_model tmp files

Fixed issue #256 using nibabel.InTemporaryDirectory().

I added the use of memmaps for the chunking the input data. It should not use more memory than peaks_from_model(... parallel=False). However, there is no test to validate that. Do you have any idea to test it?

We wiil have issue merging with PR https://github.com/nipy/dipy/pull/263. I can redo the changes once merged.