[Dipy] B encoding

Hi everyone, Given the recent conversations we had at our telecon/lab-meeting, I thought people might be interested in checking out this issue on the BIDS repo: https://github.com/bids-standard/bids-specification/issues/349 about storing b-values/b-vectors (and their generalizations) and the following PR: https://github.com/bids-standard/bids-specification/pull/352. In the meantime, do we want to add support for the mrtrix '.b' file format described? Cheers, Ariel