[r] Make SOMAs and X layers selectable when converting to a Seurat object

[aaronwolen]

This PR adds a somas argument to SOMACollection's to_seurat() method that allows you to select a subset of SOMAs and, optionally, X layers when converting to a Seurat object.

You can pass either a vector of SOMA names

soco$to_seurat(somas = c("SOMA1", "SOMA2"))

or a named list of character vectors, where each element corresponds to a SOMA name and the value is a character vector of X layers from that SOMA to include in the corresponding Seurat Assay:

soco$to_seurat(somas = list(SOMA1 = c("counts", "data"), SOMA2 = "counts"))

[shortcut-integration]

This pull request has been linked to Shortcut Story #23450: Make SOMAs and X layers selectable when converting to Seurat.