Update code in dipy/reconst (PEP8)
Update code in dipy/reconst (PEP8)

Using `pycodestyle` output, the file `dipy/reconst/dsi.py` was
updated to pass `pycodestyle` check

Signed-off-by: Antonio Ossa <1ecf3d2f96b6e61cf9b68f0fc294cab57dc5d597@uc.cl>

FrancoisRheaultUS/dipy,nilgoyyou/dipy,FrancoisRheaultUS/dipy,nilgoyyou/dipy