Warn or message when recycling scalar as Idents

[rharao]

I've occasionally seen users get confused when they do something like Idents(foo) <- "bar", expecting the Idents of foo to take the value of a column bar that doesn't exist in the metadata. I believe it would help users if this were called out.

Here is my idea:

obj <- pbmc_small
levels(obj)
## [1] "0" "1" "2"
Idents(obj) <- "group"
## Warning: 'group' not found in this object's metadata. All Idents are now the string 'group'.
## Set the Idents to `I('group')` to bypass this warning.
levels(obj)
## [1] "group"
colnames(obj[[]])
## [1] "orig.ident"      "nCount_RNA"      "nFeature_RNA"    "RNA_snn_res.0.8" "letter.idents"   "groups"         
## [7] "RNA_snn_res.1"
Idents(obj) <- "groups"
levels(obj)
## [1] "g2" "g1"
Idents(obj) <- I("group")
levels(obj)
## [1] "group"

Would this be welcome as a PR?