Serialize t-SNE

[alxvth]

• New: Save and load t-SNE plugin to projects
  • This allows continuing gradient descent and re-initializing embedding without re-computing KNNs after loading
  • This is optional - a user has to tick a small box to save the probability distribution. The UI and settings are always saved to the project, meaning that upon loading you can always see the number of iterations, perplexity, etc., used to compute the t-SNE, but the "Continue" button is disabled
• New: option to re-initialize an embedding (randomly) without re-computing the KNNs
• Improved: Move (instead of copy) data from plugin to t-SNE Analysis if it's not longer used in the plugin
• Fixed: Saving the HSNE hierarchy to disk is now opt-in instead of the default (for faster re-computation, this is separate from saving to projects)

[alxvth]

The force-pushes come from rebasing to the current master