Merge pull request #159 from matthew-brett/use-eigh-for-bmatrix

RF: Use eigh for bmatrix

Assert that B-matrix is symmetrical and use eigh to make sure we get real values.
NF+TST: utility to get b_value, b_vector from q

Trivial utility to set b_value to zero when norm of q is very small.
NF+TST: add b_value, b_vector to dicomwrappers

Use new ``q2bg`` function to set b_value and b_vector properties on
dicom wrapper objects.
RF+API: return 0, 0, 0 b_vector for b_value of 0

Before this commit we returned nan, nan, nan for zero b_value.  Now we
return (0, 0, 0).  Maybe this is more likely what people are expecting.