fix: refinements to Table schema for parse/unparser

schema/text/Table.schema.yaml

@@ -2,9 +2,8 @@ title: Table
 '@id': schema:Table
 $extends: ../CreativeWork.schema.yaml
 role: primary
-status: experimental
-description: |
-  A table within an executable document.
+status: unstable
+description: A table.
 properties:
   rows:
     '@id': stencila:rows
@@ -21,4 +20,4 @@ properties:
     items:
       $ref: TableCell.schema.yaml
 required:
-  - cells
+  - rows

schema/text/TableCell.schema.yaml

@@ -1,12 +1,11 @@
 title: TableCell
 '@id': stencila:TableCell
+$extends: ../Thing.schema.yaml
 role: secondary
 status: experimental
 description: |
   A cell within a `Table`.
 properties:
-  type:
-    enum: [TableCell]
   name:
     '@id': schema:name
     description: |
@@ -53,5 +52,4 @@ properties:
     items:
       $ref: InlineContent.schema.yaml
 required:
-  - position
   - content

schema/text/TableRow.schema.yaml

@@ -1,16 +1,15 @@
 title: TableRow
 '@id': stencila:TableRow
+$extends: ../Thing.schema.yaml
 role: secondary
 status: experimental
-description: |
-  A row within a `Table`.
+description:  A row within a Table.
 properties:
   cells:
     '@id': stencila:cells
-    description: |
-      An array of cells in the table.
+    description: An array of cells in the row.
     type: array
     items:
       $ref: TableCell.schema.yaml
 required:
-  - value
+  - cells