Make package installable

[willtownes]

Copying over comments from @giovp in issue willtownes/nsf-paper#2

• make it pip installable pip install -e .
• minimal pre-commit checks (black, flake8) for code quality
• maybe tox for tests? although I guess pytest already solution.

Then pypi publishing and docs (if planned) could be done later on.

I guess what I think would be cool to have eventually is something like this

import nsf
import scanpy as sc

# maybe some preprocessing e.g. data splitting etc.
nsf.model(adata, n_factors=..., kernel_type=..., idx_points=..., ...)
sc.pl.spatial(adata, color="factor1", ...)