made IGroupMaker (axis.i) a valid Sequence

(added __len__ and check that key < len(axis) in IGroupMaker)

Author: gdementen

larray/core/group.py

@@ -711,8 +711,13 @@ def __init__(self, axis):
         self.axis = axis
 
     def __getitem__(self, key):
+        if isinstance(key, (int, np.integer)) and not isinstance(self.axis, ABCAxisReference) and key >= len(self.axis):
+            raise IndexError("{} is out of range for axis of length {}".format(key, len(self.axis)))
         return IGroup(key, None, self.axis)
 
+    def __len__(self):
+        return len(self.axis)
+
 
 # We need a separate class for LGroup and cannot simply create a new Axis with a subset of values/ticks/labels:
 # the subset of ticks/labels of the LGroup need to correspond to its *Axis* indices