Use ceremonial boundaries for UK counties

[lonvia]

In a recent discussion on the forum there was the suggestion to use boundary=ceremonial for county level boundaries and only fall back to admin boundary 6 when there is no ceremonial boundary. In theory something like this:

{ "countries" : [ "gb" ],
  "tags" : {
      "boundary" : {
          "administrative6" : 11,
          "ceremonial": 12
      }
  }
},

for address levels should do the trick. In practice, it seems to trigger some odd behaviour when computing the address rank. There might also be some issues with ceremonial and admin boundaries being almost but not quite the same, i.e. we do not get proper contains hierarchies with this arrangement. Needs more investigation and some BDD tests.