BF,RF: Fix bug when using --jitter and --seeds-per-voxel; adjust --save-overlay option.

This MR fixes a bug which would occur when using --jitter and --seeds-per-voxel N with N greater than 1, and would result in an IndexError.

The --save-overlay option (-so for short) has also been adjusted - previously it was configured with nargs='*', so would consume any number of values, e.g.:

fsl_streamlines -so a b c ...

However, the main input argument is configured similarly, with nargs='+', which restricts how --save-overlay can be specified.

To work around this conflict, --save-overlay has been adjusted so that it only accepts one value, but can be specified more than once, e.g.:

fsl_streamlines -so a -so b -so c ...