Added all SEPA countries (IBANs from European Union countries) and minimal test coverage for countries

lib/iban_ex/country/gr.ex

@@ -0,0 +1,35 @@
+defmodule IbanEx.Country.GR do
+  @moduledoc """
+  Greece IBAN parsing rules
+
+  ## Examples
+
+    iex> %IbanEx.Iban{country_code: "GR", check_digits: "16", bank_code: "011", branch_code: "0125", national_check: nil, account_number: "0000000012300695"}
+    iex> |> IbanEx.Country.GR.to_string()
+    "GR 16 011 0125 0000000012300695"
+
+  """
+
+  @size 27
+  @rule ~r/^(?<bank_code>[0-9]{3})(?<branch_code>[0-9]{4})(?<account_number>[0-9A-Z]{16})$/i
+
+  use IbanEx.Country.Template
+
+  @impl IbanEx.Country.Template
+  @spec to_string(Iban.t()) :: binary()
+  @spec to_string(Iban.t(), binary()) :: binary()
+  def to_string(
+        %Iban{
+          country_code: country_code,
+          check_digits: check_digits,
+          bank_code: bank_code,
+          branch_code: branch_code,
+          national_check: _national_check,
+          account_number: account_number
+        } = _iban,
+        joiner \\ " "
+      ) do
+    [country_code, check_digits, bank_code, branch_code, account_number]
+    |> Enum.join(joiner)
+  end
+end