Add scalar mean reducer (fixes #184)

climix/index_functions/support.py

@@ -22,6 +22,11 @@ SUPPORTED_REDUCERS = [
     'mean',
 ]
 
+
+def scalar_mean(*args):
+    return float(np.mean(args))
+
+
 SCALAR_OPERATORS = {
     '<': operator.lt,
     '>': operator.gt,
@@ -32,6 +37,7 @@ SCALAR_REDUCERS = {
     'min': min,
     'max': max,
     'sum': sum,
+    'mean': scalar_mean,
 }