Add SubdivisionContainment

Hey there, thanks for this great plugin. Just cam across the use case to need to subdivisionContainment. Easy access through the gem would be great.

e.g.:

module Cldr
  module Export
    module Data
      class SubdivisionsContainment < Base
        def initialize(*)
          super(nil)
          update(subdivision_containment: subdivision_containment)
        end

        def subdivision_containment
          @subdivision_containment ||= doc.xpath("supplementalData/subdivisionContainment/subgroup").each_with_object(
            Hash.new { |h, k| h[k] = { contains: [] } }
          ) do |territory, memo|
            territory_id = territory.attribute("type").value
            children = territory.attribute("contains").value.split(" ")

            memo[territory_id][:contains].concat(children)
            memo[territory_id][:contains].sort!
          end
        end
      end
    end
  end
end 

Start by locating the existing Cldr::Export::Data classes and how the gem exposes exported data. Then inspect the CLDR supplementalData/subdivisionContainment/subgroup path and the proposed SubdivisionsContainment class in the issue. Done means subdivision containment is accessible through the gem and its behavior is covered by the project's relevant tests.