Fix nans in gfa

I've been wanting to fix this for months, I just never got to it. `fit.gfa` not returns 0 instead of nan when all the SH coefficients are 0 (almost exclusively in masked voxels). This should also get rid of one of the annoying warnings during the tests :).
Do you want to check explicitly where all coefficients are zero?  If so, you should use ``np.all(coef_sq == 0)``.  What you do here looks right to me, though, even though it doesn't match the comment.