Allow discontiguous out arrays in `skimage.morphology.grey`

Fixes #1216.
```python
In [1]: x = np.random.randint(0, 10, size=(10, 10))

In [2]: y = x[::2]

In [3]: y[:] = np.ascontiguousarray(y)

In [4]: y.flags
Out[4]: 
  C_CONTIGUOUS : False
  F_CONTIGUOUS : False
  OWNDATA : False
  WRITEABLE : True
  ALIGNED : True
  UPDATEIFCOPY : False
```

"I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a [suggestion]."

=P