DTI `min_signal`

This should deal with issues raised here: 

https://github.com/nipy/dipy/pull/496

without reverting. Still needs a bit of a refactor, to remove spurious checks and references to `min_signal`
Because I changed the data used. This was previously using the b0-less
dsivoxels.


On Fri, Dec 12, 2014 at 10:51 AM, Eleftherios Garyfallidis <
notifications@github.com> wrote:
>
> In dipy/reconst/tests/test_dti.py
> <https://github.com/nipy/dipy/pull/498#discussion-diff-21764521>:
>
> >      dm = dti.TensorModel(gtab, 'LS')
> >      dtifit = dm.fit(data[0, 0, 0])
> > -    assert_equal(dtifit.fa < 0.5, True)
> > +    assert_equal(dtifit.fa < 0.9, True)
>
> Why did you change the value here from 0.5 to 0.9?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/nipy/dipy/pull/498/files#r21764521>.
>