Clean up anchor line logic

[glennhickey]

The taffy view -s / --repeatCoordinatesEveryNColumns option as originally defined doesn't help much for indexing (as currently implemented). This is because it doesn't make any guarantees about producing "anchor lines" (where each coordinate is specified without needing information from previous lines).

I hacked it make an anchor line every -s bases on the first row in order to get indexing working. But it would be a good idea going forward, I think, to refactor it to explicitly make anchor lines (and forget about non-reference rows).