Fix SpellLength (closes #104)

Fixes two bugs in SpellLength:

* Remove superfluous kwargs
* Add proper masking

climix/index_functions.py

@@ -189,13 +189,17 @@ class SpellLength:
 
     def call_func(self, data, axis, **kwargs):
         axis = normalize_axis(axis, data.ndim)
-        return np.apply_along_axis(self,
-                                   axis=axis, arr=data, **kwargs)
+        mask = np.ma.getmaskarray(data).any(axis=axis)
+        res = np.apply_along_axis(self, axis=axis, arr=data)
+        res = np.ma.masked_array(np.ma.getdata(res), mask)
+        return res.astype('float32')
 
     def lazy_func(self, data, axis, **kwargs):
         axis = normalize_axis(axis, data.ndim)
-        return da.apply_along_axis(self,
-                                   axis=axis, arr=data, **kwargs)
+        mask = da.ma.getmaskarray(data).any(axis=axis)
+        res = da.apply_along_axis(self, axis=axis, arr=data)
+        res = da.ma.masked_array(da.ma.getdata(res), mask)
+        return res.astype('float32')
 
     def __call__(self, raw_data):
         data = self.condition(raw_data, self.threshold.data)