Set key when creating Seurat objects from Assays

[mojaveazure]

Fix issue where keys aren't being set when creating a Seurat object from an Assay
CreateSeuratObject.Assay now checks that the key of an assay has length and nchar
If not, creates a key from the assay name

[mojaveazure]

Test case with Signac

> library(Seurat)
Attaching SeuratObject
> library(Signac)
> packageVersion("SeuratObject")
[1] ‘4.0.3.9000’
> packageVersion("Signac")
[1] ‘1.4.0’
> data("atac_small")
> atac_small
An object of class Seurat 
1323 features across 100 samples within 3 assays 
Active assay: peaks (323 features, 323 variable features)
 2 other assays present: bins, RNA
 2 dimensional reductions calculated: lsi, umap
> counts <- GetAssayData(atac_small, assay = "peaks", slot = "counts")
> chrom_assay <- CreateChromatinAssay(counts)
> obj <- CreateSeuratObject(chrom_assay, assay = 'peaks')
Warning: Keys should be one or more alphanumeric characters followed by an underscore, setting key from peaks to peaks_
> Key(obj)
   peaks 
"peaks_" 

[timoast]

Looks good, and fixes the issues here: stuart-lab/signac#872