Changes:
- Use ``__slots__`` for ``Layers``, ``Polygons``, and ``Parameters`` ([57](https://github.com/loganbvh/superscreen/pull/57)).
- Add affine transformations for ``Polygon`` and ``Device``
([59](https://github.com/loganbvh/superscreen/pull/60), [`#60])<https://github.com/loganbvh/superscreen/pull/60)).
- Allow ``Parameters`` to return scalar or vector quantities ([ 61](https://github.com/loganbvh/superscreen/pull/61)).
- Allow explicitly setting ``num_cpus`` in ``solve_many()`` ([62](https://github.com/loganbvh/superscreen/pull/62)).
- Add ``SheetCurrentField`` source and move mesh generation into its own module to enable ``Polygon.make_mesh()``
([65](https://github.com/loganbvh/superscreen/pull/65)).
- Use ``scipy.linalg.lu_solve()`` in ``superscreen.solve()`` instead of ``numpy.linalg.inv()`` ([67](https://github.com/loganbvh/superscreen/pull/67)).