Tighten up reporting of entity identifiers

[siwhitehouse]

If I submit data on 3 entities which have the following identifiers:

{
    "id": "07444723", 
    "scheme": "XX-XXX"
  }

  {
    "id": "Helloimanid",
    "schemeName": "Kadie's register"
  }

  {
    "id": "07444723", 
    "scheme": "GB-COH"
  }

The following is reported:

@kd-ods has suggested that this would be more useful if:

i) The text read:

• % of registered or legal entities with an identifier (an ID plus scheme or schemeName):
• % of registered or legal entities with a 'scheme' value from Org-ID.guide:

ii) The statistical calculations were altered accordingly. So the data above would give:

• % of registered or legal entities with an identifier (an ID plus scheme or schemeName): 100%
• % of registered or legal entities with a 'scheme' value from Org-ID.guide: 33%

[siwhitehouse]

This has been copied from the (internal) Open Data Services issue http://bods.opendataservices.coop/redmine/issues/544