Add "linear scan" function

[oubiwann]

Treat the image data as a boustrophedon

• start at the beginning of the data
• when the max width is hit, starting reading the adjacent point, one row down
• when the min width is hit, go down another row, and repeat

To repeat indefinitely, one could:

1. simply reverse direction at the end / beginning, or
2. find all points in the image which have a value the same as the last / first point, and for each of those points, save the neighbouring points in a list, then randomly select from those when one gets to the end / beginning

Depends on:

• Add support for finite data traversals #60