Superscreen

Added `Solution.vector_potential_at_position()`, which calculates the magnetic vector potential anywhere in space due to current flowing in a `Device`.

**Note**: I accidentally named this tag `v0.6.6` instead of `v0.6.0`.

What's Changed
* Vector potential by loganbvh in https://github.com/loganbvh/superscreen/pull/73


**Full Changelog**: https://github.com/loganbvh/superscreen/compare/v0.5.0...v0.6.6

What's Changed
* Fix k space coordinates in pearl_vortex by loganbvh in https://github.com/loganbvh/superscreen/pull/74


**Full Changelog**: https://github.com/loganbvh/superscreen/compare/v0.6.6...v0.6.1

Changes:
- Add gradient(Lambda) term and clarify documentation about the model in the context of inhomogeneous films (https://github.com/loganbvh/superscreen/pull/72).

Changes

- Remove support for Python 3.6, which has reached [end-of-life](https://www.python.org/downloads/release/python-3615/) (https://github.com/loganbvh/superscreen/pull/69).

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)).

First development release, created to activate the Zenodo archive for the repository.