commands.py: Introduce a real select_file() method

This method, like select_adf(), only selects a single file ID
and unlike select_path() returns the actual status words returned by the
card.

Change-Id: I8bc86654c6d79f2428e196cc8a401e12d93a676b
diff --git a/pySim/commands.py b/pySim/commands.py
index d6159ea..2fb1041 100644
--- a/pySim/commands.py
+++ b/pySim/commands.py
@@ -115,10 +115,13 @@
 		if type(dir_list) is not list:
 			dir_list = [dir_list]
 		for i in dir_list:
-			data, sw = self._tp.send_apdu_checksw(self.cla_byte + "a4" + self.sel_ctrl + "02" + i)
+			data, sw = self.select_file(i)
 			rv.append(data)
 		return rv
 
+	def select_file(self, fid):
+		return self._tp.send_apdu_checksw(self.cla_byte + "a4" + self.sel_ctrl + "02" + fid)
+
 	def select_adf(self, aid):
 		aidlen = ("0" + format(len(aid) // 2, 'x'))[-2:]
 		return self._tp.send_apdu_checksw(self.cla_byte + "a4" + "0404" + aidlen + aid)