Cadnano2

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* Code: Reorganized to flatten module structure
* Code: Bugfixes for issues 107, 108, 112
* GUI: Legacy-style Sliceview for lattice designs
* Installer: Update to PyQt 5.9.3

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* API: Oligo.isLoop() → isCircular()
* Code: flake8 linting (ignoring E226,E266,E731,E501)
* Code: Bugfixes for issues 94, 95, 105
* Docs: Scripting examples
* Docs: How to build sphinx docs
* GUI: Updated styles to better match cadnano2
* GUI: Improvements to sliceview grid
* Installer: Update to PyQt 5.8.2
* Installer: Windows shortcut icon
* I/O: v3 oligo property "is_loop" → "is_circular"

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Major changes and new features since cadnano 2:

* Installer: Distribution as a Python package
* Design/GUI: Helices no longer constrained to lattices
* Design/GUI: Added support for "abstract" sequences
* Design/GUI: Added support for parallel crossovers
* Code: Updated from Python 2 → 3
* Code: Updated from PyQt4 → PyQt5
* Code: Rewrote underlying data model
* Code: Better stability
* GUI: Improved hinting across views
* GUI/Installer: Removed Maya plugin code
* I/O: New file format
* I/O: Easier scripting via command-line mode
* I/O: Export to STL (experimental)

Unreleased
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* GUI: 3D view
* I/O: Export to PDB (experimental)

* Migrated to PyQt6
* Packaged for PyPI and pip install

* Fixed circular scaffold highlighting when opening from an nno file.
* OSX: Reconnected objc bridge so files open via double-click or dock icon drop.
* Fixed bug in display of square-lattice crossover locations.
* Square lattice canvas extension now happens in 32-base steps.
* Updated icon for AddSeq Tool.
* Moved Frame button to the View menu and moved AutoStaple button to the top toolbar.

* Undo/redo.
* Unified interface for square-lattice and honeycomb-lattice parts. The new workflow is to open a document and then click the "Honeycomb" or "Square" button on the top toolbar to create a new part for editing.
* Stored user preferences.
* Scroll to zoom (e.g. via mousewheel). Command+mouse-drag to pan.
* Pencil tool: Left-click to create or destroy staple or scaffold. Right-click to create a forced crossover.
* DNA sequences are displayed live in the interface.
* Export button: creates a staple csv file, replacing the old SeqTool copy-to-clipboard dialog.
* Frame button: zoom-to-fit of path view.
* New file format (.nno extension)
* Loop is renamed to Insert. Its size can be edited in place via its numeric label.
* Multiple helices can be reordered by selecting the helix labels (leftmost orange circle) and dragging the group.
* Delete-Last button removed. To delete a helix, selecting its label and press delete.
* 3D view is removed. See planned features for future replacement.