Anatomical Structures, Cell Types, plus Biomarkers (ASCT+B) table for Brain, v1.6

Brain v1.6 is a minor update of the table correcting PCL labels for precursor oligodendricytes and updates citation formats. [Brain v1.5](https://cdn.humanatlas.io/hra-releases/v2.0/asct-b/asct-b-allen-brain.csv) is a minor update of the table that provides parent cell type mappings to Cell Ontology for the specific cell types that were added to [Brain v1.4](https://cdn.humanatlas.io/hra-releases/v1.4/asct-b/asct-b-allen-brain.csv), but do not yet have Cell Ontology (CL) or Provisional Cell Ontology (PCL) IDs. More of the brain anatomy was also mapped to the Uberon multispecies anatomy ontology based on exact matching synonym terms.