Add 'composed entity' value to the EntityType enum.

Add a new value 'composed entity' to the EntityType enumeration to
indicate that an ID is intended to represent a composed (aka "complex",
aka "post-coordinated") entity that involves several individual
entities.

closes #402

examples/schema/composite-entities.sssom.tsv

@@ -0,0 +1,12 @@
+#curie_map:
+#  HP: http://purl.obolibrary.org/obo/HP_
+#  MONDO: http://purl.obolibrary.org/obo/MONDO_
+#  MP: http://purl.obolibrary.org/obo/MP_
+#  SCHEMA: http://example.org/schema
+#mapping_set_id: https://w3id.org/sssom/commons/examples/composite-entities.sssom.tsv
+#license: https://creativecommons.org/publicdomain/zero/1.0/
+#comment: This is an example file for the SSSOM for illustration only. Its contents are entirely fabricated.
+subject_id	predicate_id	object_id	mapping_justification	subject_type
+SCHEMA:0001/(disease:'MONDO:0005148',phenotype:'HP:0009124')	skos:exactMatch	MP:0000003	semapv:ManualMappingCuration	composed entity
+SCHEMA:0001/(disease:'MONDO:0005149',phenotype:'HP:0008551')	skos:exactMatch	MP:0000018	semapv:ManualMappingCuration	composed entity
+SCHEMA:0001/(disease:'MONDO:0005150',phenotype:'HP:0000411')	skos:exactMatch	MP:0000018	semapv:ManualMappingCuration	composed entity

src/sssom_schema/schema/sssom_schema.yaml

@@ -45,7 +45,7 @@ enums:
         meaning: rdfs:Class
       rdfs literal:
         meaning: rdfs:Literal
-        description: This value indicate that the entity being mapped is not a semantic entity with a distinct identifier, but is instead represented entirely by its literal label. This value MUST NOT be used in the predicate_type slot.
+        description: This value indicates that the entity being mapped is not a semantic entity with a distinct identifier, but is instead represented entirely by its literal label. This value MUST NOT be used in the predicate_type slot.
         see_also:
         - https://mapping-commons.github.io/sssom/spec-model/#literal-mappings
         - https://github.com/mapping-commons/sssom/issues/234
@@ -54,6 +54,11 @@ enums:
         meaning: rdfs:Datatype
       rdf property:
         meaning: rdf:Property
+      composed entity:
+        description: This value indicates that the entity ID does not represent a single entity, but a composite involving several individual entities. This value MUST NOT be used in the predicate_type slot. This specifications does not prescribe how an ID representing a composite entity should be interpreted; this is left at the discretion of applications.
+        see_also:
+        - https://github.com/mapping-commons/sssom/issues/402
+        - https://github.com/mapping-commons/sssom/blob/master/examples/schema/composite-entities.sssom.tsv
 
   predicate_modifier_enum:
     permissible_values: