Add key is_about to oemetadata

Make necessary changes to introduce the key is_about. The key should be used to describe the column header in ontology terms with a URI.
See: #52

Author: chrwm

metadata/latest/example.json

@@ -210,24 +210,28 @@
                         "name": "id",
                         "description": "Unique identifier",
                         "type": "serial",
+                        "is_about": null,
                         "unit": null
                     },
                     {
                         "name": "year",
                         "description": "Reference year",
                         "type": "integer",
+                        "is_about": null,
                         "unit": null
                     },
                     {
                         "name": "value",
                         "description": "Example value",
                         "type": "double precision",
+                        "is_about": null,
                         "unit": "MW"
                     },
                     {
                         "name": "geom",
                         "description": "Geometry",
                         "type": "geometry(Point, 4326)",
+                        "is_about": null,
                         "unit": null
                     }
                 ],

metadata/latest/metadata_key_description.md

@@ -73,6 +73,7 @@ This pages describes the OEP Metadata version 1.4.1 You can have a look at an em
 | 14.6.1.1 | name | Name string unique within its scope. | year |
 | 14.6.1.2 | description | Free-text describing the field. | Reference year for which the data were collected. |
 | 14.6.1.3 | type | Data type of the field. In case of a geom-column in a database, also indicate the shape and  CRS. | geometry(Point, 4326) |
+| | is_about | Ontology URI to describe the column header | |
 | 14.6.1.4 | unit | Unit, preferably SI-Unit, that values in this field are mapped to. If 'unit' doesn't apply to a field, use 'null' | MW |
 | 14.6.2 | primaryKey | A primary key is a field or set of fields that uniquely identifies each row in the table. It's recorded as a list of strings, since it is possible to define the primary key as made up of several columns. | id |
 | 14.6.3 | foreignKeys | A foreign key is a field that refers to a column in another table. | |

metadata/latest/schema.json

@@ -470,6 +470,13 @@
                                                 "null"
                                             ]
                                         },
+                                        "is_about": {
+                                            "description": "Ontology URI to describe the column header",
+                                            "type": [
+                                                "string",
+                                                "null"
+                                            ]
+                                        },
                                         "unit": {
                                             "description": "Unit, preferably SI-Unit, that values in this field are mapped to. If \"unit\" doesn't apply to a field, use \"null\". Example: MW",
                                             "type": [

metadata/latest/template.json

@@ -114,12 +114,14 @@
                         "name": null,
                         "description": null,
                         "type": null,
+                        "is_about": null,
                         "unit": null
                     },
                     {
                         "name": null,
                         "description": null,
                         "type": null,
+                        "is_about": null,
                         "unit": null
                     }
                 ],