RF: add check for symmetry of B matrix, use eigh

Some dicoms that Satra tested on were giving complex q vectors.  Add
check for symmetry of input B matrix, and use symmetry to guarantee
real-valued eigenvectors using ``np.linalg.eigh``.
RF+TST: adjust eigenvalue threshold slightly

Adjust default eigenvalue threshold for negative eigenvalues according
to the MATLAB algorithm - although it will have a tiny effect.

Test the default threshold, and setting threshold.
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.