v0.4.0

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.