performance : allow to have labels in cache

[davidchavalarias]

Computing labels positions and ordering takes time for large graphs even when the number of available labels is low.

The user experience would be much improved if we had the possibility to put the result of labels computation in cache for reuse across same different displays of the same graph.

[donghaoren]

Are you using the online "Load Data" version (https://apple.github.io/embedding-atlas/app/)? If you use the command line tool it should have a label cache, but the cache is not yet enabled for the online version.