Peak from model issue #253

This PR should fix issue https://github.com/nipy/dipy/issues/253.

The problem is in the call to np.linalg.pinv, we didn't figure out why this happens on the buildbot. Nevertheless, this should fix the problem since pinv is no more called in the peaks_from_model_parallel. We added 2 optional parameters (B, invB) to peak_from_model, and give it pre-computed to the parallel function.

additionnaly,

- if B is already computed before calling peak_from_model, no need to recompute it
- if not, B is only computed once in the parallel version, instead of once per process