Bug when EFG tensor is zero

[tfgg]

Following reported by Andrew Morris:

I've had magres-format generate an error.
"/home/ajm255/lib/python2.7/site-packages/magres/constants.py:17:
RuntimeWarning: invalid value encountered in double_scalars
return (eval[2], (eval[0]-eval[1])/eval[2])"

I know very little Python, but I seem to be able to generate it when
an efg tensor is exactly zero, see attached. Presumably this is
causing numpy to have problems finding its eigenvalues? In
constants.py: sorted_evals(mat)?