fix: ATAC optimization part 2

[kaloster]

This is part 2 of this optimization PR

• Update typo in example dataset name
• Added cache per static object
• Coverage endpoint can now be queried for multiple cell_types
• Added FE updates to accommodate the revised endpoint

atac/coverage?gene_name=MYC&genome_version=hg38&cell_types=endothelial cell, leukocyte

and will return something like:

{
  "chromosome": "chr1",
  "coverageByCellType": {
    "endothelial cell": [[0.0, 1000000.0, 1000500.0], ...],
    "leukocyte": [[0.1, 1000000.0, 1000500.0], ...]
  },
  "geneInfo": [...]
}