Flexinet

- Add independent encoder / decoder modules

**Full Changelog**: https://github.com/mvinyard/flexinet/compare/v0.0.2...v0.0.3

- Expanded MLP instantiation
- `NN` class
- `flexinet.models.compose_nn_sequential()` to automatically flexibly instantiation a `NN` given easily-parsed/understood configuration arguments.

- Example implementation using a vanilla linear VAE - mostly re-implemented from this useful example: [link](https://debuggercafe.com/getting-started-with-variational-autoencoder-using-pytorch/)
- Since I am a single-cell person, there is a `read_h5ad` function for [anndata](https://anndata.readthedocs.io/en/stable/)
- Some initial debugging has been performed but further testing required.