add optional id to a theme concept

[tomkralidis]

Adds an optional id to a theme concept. Example:

        {
            "concepts": [
                {
                    "id": "adbef130576",
                    "name": "atmosphericComposition",
                    "url": "https://wis.wmo.int/2012/codelists/WMOCodeLists.xml#WMO_CategoryCode_atmosphericComposition"
                }
            ],
            "scheme": "https://wis.wmo.int/2012/codelists/WMOCodeLists.xml#WMO_CategoryCode"
        }

[pvretano]

12-DEC-2022: After SWG discussion the structure of the "concepts" item will be change to have "id", "title", "description" and "url". The "id" is the identifier of the concept. The "title" is the human-readable title for the concept. The "description" is a human-readable narrative about the concept and the "url" is a reference to the concept. The "id" is mandatory, the rest are options. @tomkralidis will make the necessary revisions.

[tomkralidis]

FYI PR has been updated accordingly and ready for review.

[tomkralidis]

Updated example (based on current PR):

        {
            "concepts": [
                {
                    "id": "atmosphericComposition",
                    "title": "Atmospheric composition",
                    "url": "https://wis.wmo.int/2012/codelists/WMOCodeLists.xml#WMO_CategoryCode_atmosphericComposition"
                }
            ],
            "scheme": "https://wis.wmo.int/2012/codelists/WMOCodeLists.xml#WMO_CategoryCode"
        }