Resampling of nD arrays

Current api :

```py
In [1]: import numpy as np
In [2]: from skimage.transform._warps import downscale_local_means
In [3]: a = np.arange(16).reshape(4, 4)
In [4]: a
Out[4]: 
array([[ 0,  1,  2,  3],
       [ 4,  5,  6,  7],
       [ 8,  9, 10, 11],
       [12, 13, 14, 15]])
In [5]: downscale_local_means(a, (2, 2))
Out[5]: 
array([[  2.5,   4.5],
       [ 10.5,  12.5]])
In [6]: 
In [6]: downscale_local_means(a, (2, 3))
Out[6]: 
array([[  3.        ,   1.66666667],
       [ 11.        ,   4.33333333]])
In [7]: from skimage.measure._sum_blocks import sum_blocks

In [8]: sum_blocks(a, (2, 2))
Out[8]: 
array([[10, 18],
       [42, 50]])
```