feat: add Expression (#10)

* feat: add Expression
* chore: remove .readthedocs.yaml (#11)
  * not used at the moment

schema/gks-common/common-source.yaml

@@ -139,3 +139,22 @@ $defs:
           type: string
     heritableRequired:
       - type
+
+  Expression:
+    type: object
+    maturity: draft
+    description: >-
+      Representation of a variation by a specified nomenclature or syntax for a Variation object.
+      Common examples of expressions for the description of molecular variation include the HGVS
+      and ISCN nomenclatures.
+    properties:
+      syntax:
+        type: string
+        enum: ["hgvs.c", "hgvs.p", "hgvs.g", "hgvs.m", "hgvs.n", "hgvs.r", "iscn", "gnomad", "spdi"]
+      value:
+        type: string
+      syntax_version:
+        type: string
+    required:
+      - syntax
+      - value

schema/gks-common/def/Expression.rst

@@ -0,0 +1,28 @@
+**Computational Definition**
+
+Representation of a variation by a specified nomenclature or syntax for a Variation object. Common examples of expressions for the description of molecular variation include the HGVS and ISCN nomenclatures.
+
+    **Information Model**
+
+    .. list-table::
+       :class: clean-wrap
+       :header-rows: 1
+       :align: left
+       :widths: auto
+
+       *  - Field
+          - Type
+          - Limits
+          - Description
+       *  - syntax
+          - string
+          - 1..1
+          - 
+       *  - value
+          - string
+          - 1..1
+          - 
+       *  - syntax_version
+          - string
+          - 0..1
+          - 

schema/gks-common/json/Expression

@@ -0,0 +1,35 @@
+{
+   "$schema": "https://json-schema.org/draft/2020-12/schema",
+   "$id": "https://w3id.org/ga4gh/schema/gks-common/1.x/json/Expression",
+   "title": "Expression",
+   "type": "object",
+   "maturity": "draft",
+   "description": "Representation of a variation by a specified nomenclature or syntax for a Variation object. Common examples of expressions for the description of molecular variation include the HGVS and ISCN nomenclatures.",
+   "properties": {
+      "syntax": {
+         "type": "string",
+         "enum": [
+            "hgvs.c",
+            "hgvs.p",
+            "hgvs.g",
+            "hgvs.m",
+            "hgvs.n",
+            "hgvs.r",
+            "iscn",
+            "gnomad",
+            "spdi"
+         ]
+      },
+      "value": {
+         "type": "string"
+      },
+      "syntax_version": {
+         "type": "string"
+      }
+   },
+   "required": [
+      "syntax",
+      "value"
+   ],
+   "additionalProperties": false
+}