Make identifier singular (remove possibility for a list of identifiers) for Locus and Allele

So far I see no values with such data types used at all in dandi archive so we should not
break anything

An alternative would be to just make it a List[Identifier] always and not
bother allowing for the dance of singular-vs-plural.

Author: yarikoptic

dandischema/models.py

@@ -1340,7 +1340,7 @@ class PublishActivity(Activity):
 
 
 class Locus(DandiBaseModel):
-    identifier: Union[Identifier, List[Identifier]] = Field(
+    identifier: Identifier = Field(
         description="Identifier for genotyping locus.",
         json_schema_extra={"nskey": "schema"},
     )
@@ -1352,7 +1352,7 @@ class Locus(DandiBaseModel):
 
 
 class Allele(DandiBaseModel):
-    identifier: Union[Identifier, List[Identifier]] = Field(
+    identifier: Identifier = Field(
         description="Identifier for genotyping allele.",
         json_schema_extra={"nskey": "schema"},
     )