pySim-shell: Reject any non-decimal PIN values

Don't even send any non-decimal PIN values to the card, but reject
them when parsing the command arguments.

Change-Id: Icec1698851471af7f76f20201dcdcfcd48ddf365

pySim/utils.py

@@ -1487,3 +1487,10 @@ def is_hexstr(instr: str) -> str:
     if len(instr) & 1:
         raise ValueError('Input has un-even number of hex digits')
     return instr
+
+def is_decimal(instr: str) -> str:
+    """Method that can be used as 'type' in argparse.add_argument() to validate the value consists of
+    an even sequence of decimal digits only."""
+    if not instr.isdecimal():
+        raise ValueError('Input must decimal')
+    return instr