crossover to same helix should be converted properly in cadnano import

[dave-doty]

In cadnano 2, create a strand with two domains on the same helix and a "crossover" between them:

This should be representable in scadnano and show up like this:

Import this design into scadnano. Instead, it interprets the two domains and the crossover as one long domain:

This cadnano file represents the above simple design, to help reproduce this error.

[tcosmo]

@dave-doty @UnHumbleBen

I have solved this issue with this PR: #214

It should not be complex to adapt in the Dart code as it was only the case to add one more case to the condition that decides whether or not to add a crossover (condition 2.) :