WIP scale_source_space():  add support for vol source spaces

Trying to adapt scale_source_space to handle volume source spaces. As far as I can tell the additional source space entries that need scaling are: 

* vox_mri_t
* src_mri_t
* interpolator

When looking at the transforms in my volume source space I created for fsaverage I was somewhat confused: 

```py
>>> pprint(vsrc['src_mri_t'])
{'from': 2001,
 'to': 5,
 'trans': array([[ 0.01,  0.  ,  0.  , -0.08],
       [ 0.  ,  0.01,  0.  , -0.12],
       [ 0.  ,  0.  ,  0.01, -0.08],
       [ 0.  ,  0.  ,  0.  ,  1.  ]])}
>>> pprint(vsrc['vox_mri_t'])
{'from': 2001,
 'to': 5,
 'trans': array([[-0.001     ,  0.        ,  0.        ,  0.12800001],
       [ 0.        ,  0.        ,  0.001     , -0.12800001],
       [ 0.        , -0.001     ,  0.        ,  0.12800001],
       [ 0.        ,  0.        ,  0.        ,  1.        ]])}
```

Shouldn't one be the inverse of the other? I.e.

```py
>>> sp.linalg.inv(vsrc['src_mri_t']['trans'])
array([[ 100.00000224,    0.        ,   -0.        ,    8.        ],
       [   0.        ,  100.00000224,   -0.        ,   12.        ],
       [   0.        ,    0.        ,  100.00000224,    8.        ],
       [   0.        ,    0.        ,    0.        ,    1.        ]])
```
Since source space coordinates seem to be in [m] I assume the MRI is in [mm]? 

The `setup_volume_source_space()` documentation was not clear to me as to which MRI should be provided. I just used `orig.mgz`, is that maybe the problem? (@mluessi?)