Add rudimentary parsing of literals to PET driver (i-pi#341)

This is needed to pass non-strings such as None,
True, or False into the SingleStructCalculator.

I've drawn the line at floats and ints, because
it is not so easy to programmatically distinguish
them, and it's not always clear what the intended
type is supposed to be.

drivers/py/pes/pet.py

@@ -54,6 +54,15 @@ def check_arguments(self):
             if len(args) > 2:
                 for arg in args[2:]:
                     key, value = arg.split("=")
+                    lookup = {
+                        "None": None,
+                        "False": False,
+                        "True": True,
+                    }
+
+                    if value in lookup:
+                        value = lookup[value]
+
                     kwargs[key] = value
 
         else: