Download single cell datasets from different portals

[yiqisu]

To satisfy the increasing need for scRNA-seq data and other omics data such as scATAC-seq, we'd like to add new features to BEELINE to enable the automatic downloading of datasets from data portals including sfaira, Single Cell Portal (SCP), Gene Expression Omnibus (GEO), etc.

[yiqisu]

The SCP API doesn't directly provide the filtering capabilities for automatic downloads such as by omics and/or tissue type. We will integrate to BEELINE in the future.