Allow ENVI to cache COVET matrices on disk to avoid recomputing COVET and to enable larger datasets

[tddough98]

When initializing ENVI, all COVET matrices for all cells in spatial data are computed and held in memory. The COVET matrices must all fit into RAM, which limits the number of cells ENVI can handle and the number of genes used to compute COVET, and must be recomputed every time the model is initialized.

Allowing ENVI to save COVET matrices to disk after computing them and loading them in batches would

1. enable larger datasets (number of cells)
2. enables larger COVET matrices (more genes)
3. separate computing COVET matrices from training, allowing COVET matrices to be pre-computed and reused