Detect precipiation in input data, not threshold parameter (closes #39)

climix/index_functions.py

@@ -12,7 +12,7 @@ class SpellLength:
         self.reducer = reducer
 
     def prepare(self, input_cube):
-        if is_precipitation(self.threshold):
+        if is_precipitation(self.input_cube):
             change_pr_units(self.threshold,
                             new_unit=input_cube.units,
                             new_stname=input_cube.standard_name)