Ceviche

What's Changed
* Minor fix for viz.py by kdotom in https://github.com/fancompute/ceviche/pull/23
* Change deprecated `np.complex` by Jan-David-Black in https://github.com/fancompute/ceviche/pull/29

New Contributors
* kdotom made their first contribution in https://github.com/fancompute/ceviche/pull/23
* Jan-David-Black made their first contribution in https://github.com/fancompute/ceviche/pull/29

**Full Changelog**: https://github.com/fancompute/ceviche/compare/0.1.2...v0.1.3

Added multifrequency FDFD for simulating periodically modulated permittivity.

Fixed some rough edges with sparse primitives release.
Put code from older commits back in.
Made units SI.

Now the autograd primitives are abstracted from the physics. And sparse - matrix operations are defined based on their matrix indices and elements. Some changes to the FDFD class were also made for simplification.