Convert Azimuth Demo Datasets to ASCT+B tables

[DarshalShetty]

Perform the same data extraction as Azimuth's reference files (described in #129) but now with the following demo datasets:

• Human PBMC Demo Dataset(s): 11,769 PBMCs from 10x Genomics
• Human Pancreas Demo Dataset(s): Enge et al, Cell 2017 [Seurat Object]
• Human motor cortex Demo Dataset(s): Subset of M1C Data (Portal) [Seurat Object]
• Human lung Demo Dataset(s): Vieira-Braga et al, Nature Medicine 2019 [Seurat Object]; corresponding authors: Martijn C. Nawijn & Sarah A. Teichmann
• Human fetal development Demo Dataset(s): Asp et al, Cell 2019; Enge et al, Cell 2017; Lindstrom et al, Developmental Cell 2018; Cao et al, Science 2020