Per-cell data v1

Per-cell count matrices, per-cell metadata (including cell type annotation and barcode), and WormBase gene ID <-> symbol maps for each CeNGEN dataset. See README.md for the schema and a load recipe. These are large binary files hosted as release assets (not tracked in git) since they exceed git's practical size limits; the aggregate gene x neuron_type reference tables used by cengenAnnotate remain on the pins board.