Saving calculations during mosaic map & final layer activations

[skochanny]

Currently, each mosaic map generation re-calculates final layer activations even if these have been calculated previously. In order to save time, calculated final layer activations should be saved to disk and re-used if the model and tfrecords list are the same.

Proposal: record model & tfrecord list into a JSON file and associate this combination with a filename for the saved final layer activations (CSV). During subsequent calls, this file should be loaded directly instead of re-calculated

[jamesdolezal]

First implementation in 8003359

Currently, the Mosaic class will save final layer activations to both CSV and PKL (python cache file) after calculation. PKL is much faster to read/write, but cannot be read in a spreadsheet, so both are valuable. The ActivationsVisualizer class can load from either, and will also cache UMAP calculations in PKL format.

The Mosaic class still needs to be able to read from saved CSV and/or PKL format.

[jamesdolezal]

Now implemented in b26b5a4. Layer activations are cached to PKL and optionally exported to CSV during calculation, and both can be imported by future models to remove the need for additional calculations.