domain_tuple: add total_volume property

nifty5/domain_tuple.py

@@ -125,6 +125,13 @@ class DomainTuple(object):
         """
         return self._size
 
+    @property
+    def total_volume(self):
+        res = 1.
+        for d in self._dom:
+            res *= d.total_volume
+        return res
+
     @property
     def axes(self):
         """tuple of tuple of int : axis indices of the underlying domains"""

nifty5/field.py

@@ -273,11 +273,13 @@ class Field(object):
         float
             the total volume of the requested sub-domain.
         """
+        if spaces is None:
+            return self._domain.total_volume
+
         if np.isscalar(spaces):
             return self._domain[spaces].total_volume
 
-        if spaces is None:
-            spaces = range(len(self._domain))
+        # tuple of spaces given
         res = 1.
         for i in spaces:
             res *= self._domain[i].total_volume