Add issue tracker item to SSSOM datamodel (#259)

Fixes #78 

- [x] `docs/` have been added/updated if necessary
- [x] `make test` has been run locally
- [x] tests have been added/updated (if applicable)
- [x]
[CHANGELOG.md](https://github.com/mapping-commons/sssom/blob/0.9.0/CHANGELOG.md)
has been updated.

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog for SSSOM
 
+## Next
+
+- Add issue_tracker_item and issue_tracker [model elements](https://github.com/mapping-commons/sssom/pull/259).
+
 ## SSSOM version 0.13.0
 
 - The necessity of the "canonical column ordering" was downgraded from MUST to SHOULD (https://github.com/mapping-commons/sssom/pull/285)

examples/schema/issue_tracker.sssom.tsv

@@ -0,0 +1,15 @@
+#curie_map:
+#  HP: http://purl.obolibrary.org/obo/HP_
+#  MP: http://purl.obolibrary.org/obo/MP_
+#  orcid: https://orcid.org/
+#  MH_MAPPINGS_GITHUB_ISSUES: https://github.com/mapping-commons/mh_mapping_initiative/issues/
+#mapping_set_id: https://w3id.org/sssom/commons/examples/issue_tracker.sssom.tsv
+#license: "https://creativecommons.org/publicdomain/zero/1.0/"
+#creator_id: orcid:0000-0002-7356-1779
+#mapping_provider: "https://w3id.org/sssom/core_team"
+#issue_tracker: "https://github.com/mapping-commons/mh_mapping_initiative/issues"
+#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	issue_tracker_item
+HP:0009124	skos:exactMatch	MP:0000003	semapv:ManualMappingCuration	MH_MAPPINGS_GITHUB_ISSUES:9123
+HP:0008551	skos:exactMatch	MP:0000018	semapv:ManualMappingCuration	
+HP:0000411	skos:exactMatch	MP:0000021	semapv:ManualMappingCuration	MH_MAPPINGS_GITHUB_ISSUES:9125

examples/schema/issue_tracker_item.sssom.tsv

@@ -0,0 +1,14 @@
+#curie_map:
+#  HP: http://purl.obolibrary.org/obo/HP_
+#  MP: http://purl.obolibrary.org/obo/MP_
+#  orcid: https://orcid.org/
+#  MH_MAPPINGS_GITHUB_ISSUES: https://github.com/mapping-commons/mh_mapping_initiative/issues/
+#mapping_set_id: https://w3id.org/sssom/commons/examples/issue_tracker_item.sssom.tsv
+#license: "https://creativecommons.org/publicdomain/zero/1.0/"
+#creator_id: orcid:0000-0002-7356-1779
+#mapping_provider: "https://w3id.org/sssom/core_team"
+#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	issue_tracker_item
+HP:0009124	skos:exactMatch	MP:0000003	semapv:ManualMappingCuration	MH_MAPPINGS_GITHUB_ISSUES:9123
+HP:0008551	skos:exactMatch	MP:0000018	semapv:ManualMappingCuration	
+HP:0000411	skos:exactMatch	MP:0000021	semapv:ManualMappingCuration	MH_MAPPINGS_GITHUB_ISSUES:9125

src/sssom_schema/schema/sssom_schema.yaml

@@ -479,6 +479,26 @@ slots:
     examples:
       - value: https://www.wikidata.org/wiki/Q865360
         description: (the Wikidata identifier for the Jaccard index measure).
+  issue_tracker_item:
+    description: The issue tracker item discussing this mapping.
+    range: EntityReference
+    examples:
+      - value: SSSOM_GITHUB_ISSUE:166
+        description: (A URL resolving to an issue discussing a new SSSOM element request)
+    see_also: 
+      - https://github.com/mapping-commons/sssom/issues/78
+      - https://github.com/mapping-commons/sssom/pull/259
+      - https://github.com/mapping-commons/sssom/blob/master/examples/schema/issue_tracker_item.sssom.tsv
+  issue_tracker:
+    description: A URL location of the issue tracker for this entity.
+    range: uri
+    examples:
+      - value: https://github.com/mapping-commons/mh_mapping_initiative/issues
+        description: (A URL resolving to the issue tracker of the Mouse-Human mapping initiative)
+    see_also: 
+      - https://github.com/mapping-commons/sssom/issues/78
+      - https://github.com/mapping-commons/sssom/pull/259
+      - https://github.com/mapping-commons/sssom/blob/master/examples/schema/issue_tracker.sssom.tsv
   see_also:
     description: A URL specific for the mapping instance. E.g. for kboom we have a
       per-mapping image that shows surrounding axioms that drive probability. Could
@@ -527,6 +547,7 @@ classes:
     - subject_preprocessing
     - object_preprocessing
     - see_also
+    - issue_tracker
     - other
     - comment
   mapping:
@@ -573,6 +594,7 @@ classes:
     - semantic_similarity_score
     - semantic_similarity_measure
     - see_also
+    - issue_tracker_item
     - other
     - comment
     class_uri: owl:Axiom
@@ -587,6 +609,7 @@ classes:
       - mapping_set_references
       - documentation
       - homepage
+      - issue_tracker
   mapping set reference:
     description: A reference to a mapping set. It allows to augment mapping 
       set metadata from the perspective of the registry, for example, providing