make tutorial that uses chained methods

[dave-doty]

The tutorial is a bit incomplete in the sense that it doesn't show the "default" method of creating strands, which is describing their route from 5' end to 3' end. It instead uses a nifty trick of describing the regular nick and crossover pattern that works nicely for large periodic structures, but not as well for other designs.

So, make a new tutorial that uses chained methods such as

design.strand(0, 8).move(8).cross(1).move(-16).cross(2).move(8);