Explanation of the DB versions

[stemangiola]

• base (0.1): This is the first version
• 0.1.5
  • new metadata with the 0.1 cell_type_harmonised because the 0.2 had a bug
• 0.2:
  • updated tissue classification
  • update gene nomenclature
  • curated some gene distributions
  • update cell predictions, keeping same hierarchy
• 0.2.1
  • update cell hierarchy
  • dropped all 0 cells, and Inf counts