Opentsne

Changes:
- Remove `affinites` from `TSNE` construction, allow custom affinities and initialization in `.fit` method. This improves the API when dealing with non-tabular data. This is not backwards compatible.
- Add `metric="precomputed"`. This includes the addition of `openTSNE.nearest_neighbors.PrecomputedDistanceMatrix` and `openTSNE.nearest_neighbors.PrecomputedNeighbors`.
- Add `knn_index` parameter to `openTSNE.affinity` classes.
- Add (less-than-ideal) workaround for pickling Annoy objects.
- Extend the range of recommended FFTW boxes up to 1000.
- Remove deprecated `openTSNE.nearest_neighbors.BallTree`.
- Remove deprecated `openTSNE.callbacks.ErrorLogger`.
- Remove deprecated `TSNE.neighbors_method` property.
- Add and set as default `negative_gradient_method="auto"`.

Main changes:
- Build wheels for MacOS target 10.6
- Update to annoy v1.17.0, this should result in much faster multi-threaded performance

**Major changes:**
- Remove numba dependency, switch over to using Annoy nearest neighbor search. Pynndescent is now optional and can be used if installed manually.
- Massively speed-up transform by keeping reference interpolation grid fixed. Limit new points to circle centered around reference embedding.
- Implement variable degrees of freedom.

**Minor changes:**
- Add spectral initialization using diffusion maps.
- Replace cumbersome `ErrorLogger` callback with the `verbose` flag.
- Change the default number of iterations to 750.
- Add `learning_rate="auto"` option.
- Remove the `min_grad_norm` parameter.

**Bugfixes:**
- Fix case where KL divergence was sometimes reported as NaN.

In order to make usage as simple as possible and remove and external dependencies on FFTW (which needed to be installed locally before), this update replaces FFTW with numpy's FFT.