> no numpy function does this

Unless I've missed your point, that's not true:
```python
>>> import numpy as np
>>> np.__version__
'1.7.2'
>>> a = np.arange(3*4*5).reshape(3,4,5)
>>> np.mean(a, axis=(0,1))
array([ 27.5,  28.5,  29.5,  30.5,  31.5])
```