This gives a nice way to specify toeholds and extensions common in DNA strand displacement designs.

See also UC-Davis-molecular-computing/scadnano#34.

This is not supported currently in scadnano, and it will take a lot of effort to support it, since much of the logic pervading the code assumes the first and last substrands are Domain's. There's nothing requiring this in principle, but it will be a headache to change it. Also, some design decisions will have to be made along the way. 

For example, the default staple name for exporting sequences is the same as cadnano, where the staple is named after the (helix,offset) pairs of its 5' and 3' ends. We could do the same thing, where if the end of a strand is an Extension, we use the adjacent Domain's to name the staple. But little decisions like this will probably have to happen all over the place.