Pymatting

- Add `relative_discard_threshold` for `ichol` preconditioner.

- Switch back to `njit` because ahead-of-time compilation caused too many issues with installation.

- Add optimization for `estimate_alpha_cf` which should reduce computation time if most pixels in the trimap are known.
- Allow sloppy trimaps.

- Recompile ahead-of-time-compiled modules if they are out of date.
- Add a gradient weighting term to `estimate_foreground_ml`.

- Compile on first import instead of during build to simplify PyPI upload process.

- Replace just-in-time compilation with ahead-of-time compilation for faster import times.