Fix publication_date (#294)

Resolves #293 

A property (publication_date) was defined in SSSOM, but was not added to
the SSSOM mapping and mapping set data model. Since a very similar
property (mapping_date) was already defined, this change is considered a
bugfix, rather than an added feature.

src/sssom_schema/schema/sssom_schema.yaml

@@ -524,6 +524,7 @@ classes:
     - mapping_tool
     - mapping_tool_version
     - mapping_date
+    - publication_date
     - subject_match_field
     - object_match_field
     - subject_preprocessing
@@ -563,6 +564,7 @@ classes:
     - mapping_tool
     - mapping_tool_version
     - mapping_date
+    - publication_date
     - confidence
     - curation_rule
     - curation_rule_text