ENH,BF: New fsl_streamlines --seed-mask option, and bug fix

New --seed-mask option to fsl_streamlines, allowing seeds to be defined with a mask.

Fixed a bug which would cause an error when seeding from a voxel with null/zero orientation.

Change streamlining behaviour so that if only one of forward- or backward-tracking produces a streamline, the streamline is kept. The previous behaviour would discard a streamline if either forward- or backward-tracking did not produce anything.