Repair silent truncation of config override

Previously when apply_user_config was called with a float value
in user_config to be applied to a key with an int value in config,
the float value would be coerced to int potentially resulting in
truncation.

For the special case of a new float value for a config key with an int
value, the float value is now assigned directly to the config key so
no truncation occurs.

desmod/config.py

@@ -120,7 +120,10 @@ def apply_user_config(config, user_config):
         except KeyError:
             raise ConfigError('Invalid config key: {}'.format(key))
         current_type = type(current_value)
-        if not isinstance(value, current_type):
+        if not (isinstance(value, current_type) or
+                # allow new float value to replace integer default
+                #  without truncation from int to float coercion
+                (isinstance(value, float) and issubclass(current_type, int))):
             try:
                 value = current_type(value)
             except (ValueError, TypeError):