Scadnano

***Breaking change:***
Changed name of Helix.position3d to Helix.position

Changed documentation explaining meaning of Helix.position to match new interpretation in scadnano of how it translates to side view and main view display, for issue 78, which swaps the role of Helix.position.x and Helix.position.z.

The Python library itself did not do any such translation, but scripts will have to be re-written to change x and z to display properly in the scadnano web interface once issue https://github.com/UC-Davis-molecular-computing/scadnano/issues/307 is closed.

Removed need to have xlwt installed if downloading the scadnano.py file and importing locally, unless an Excel file needs to be written.

Removed support for Helix.rotation and Helix.rotation_anchor.

Removed pitch, roll, and yaw from Helix.position, and made them top-level fields in Helix.

See https://github.com/UC-Davis-molecular-computing/scadnano/blob/master/README.md for description of current data model.