[core] Log parameter that were not used after each draw

Whenever a new bar is being drawn, log out all parameters that were not
used at all. This might indicate a bug in the module, or might point to
a spelling error in the parameter name.

fixes #494

bumblebee/store.py

@@ -9,13 +9,19 @@ class Store(object):
     """Interface for storing and retrieving simple values"""
     def __init__(self):
         self._data = {}
+        self._unused = {}
 
     def set(self, key, value):
         """Set 'key' to 'value', overwriting 'key' if it exists already"""
         self._data[key] = value
+        self._unused[key] = value
+
+    def unused_keys(self):
+        return self._unused.keys()
 
     def get(self, key, default=None):
         """Return the current value of 'key', or 'default' if 'key' is not set"""
+        self._unused.pop(key, None)
         return self._data.get(key, default)
 
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