United Kingdom IBAN parsing rules

Examples

    iex>  %IbanEx.Iban{
    ...>    country_code: "GB",
    ...>    check_digits: "29",
    ...>    bank_code: "NWBK",
    ...>    branch_code: "601613",
    ...>    national_check: "06",
    ...>    account_number: "31926819"
    ...>  }
    ...>  |> IbanEx.Country.GB.to_string()
    "GB 29 NWBK 601613 31926819"