loganbvh/superscreen: v0.3.0

Changes:

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