GNU R data structures for single cell data

 Defines S4 classes for single-cell genomic data and associated
 information, such as dimensionality reduction embeddings, nearest-neighbor
 graphs, and spatially-resolved coordinates. Provides data access methods and
 R-native hooks to ensure the Seurat object is familiar to other R users. See
 Satija R, Farrell J, Gennert D, et al (2015) <doi:10.1038/nbt.3192>,
 Macosko E, Basu A, Satija R, et al (2015) <doi:10.1016/j.cell.2015.05.002>,
 and Stuart T, Butler A, et al (2019) <doi:10.1016/j.cell.2019.05.031> for
 more details.