pySim.apdu: Refactor cmd_to_dict() method

Let's factor out the "automatic processing using _tlv / _construct" as a
separate method.  This way we enable a derived class to first call that
automatic processing method, and then amend its output in a second step.

Change-Id: I1f066c0f1502020c88d99026c25bf2e283c3b4f5

pySim/apdu/__init__.py

@@ -292,6 +292,10 @@ class ApduCommand(Apdu, metaclass=ApduCommandMeta):
         if callable(method):
             return method()
         else:
+            return self._cmd_to_dict()
+
+    def _cmd_to_dict(self) -> Dict:
+        """back-end function performing automatic decoding using _construct / _tlv."""
         r = {}
         method = getattr(self, '_decode_p1p2', None)
         if callable(method):