Fix index function spell_length (fixes #91)

climix/index_functions.py

@@ -85,8 +85,8 @@ class CountOccurences:
 class SpellLength:
     def __init__(self, threshold, condition, reducer):
         self.threshold = threshold
-        self.condition = condition
-        self.reducer = reducer
+        self.condition = NUMPY_OPERATORS[condition]
+        self.reducer = NUMPY_REDUCERS[reducer]
         self.standard_name = None
         self.units = Unit('days')