slice_geometry ignores "pre"-indices

```
>> a = enmap.ones((3,100,100))
>>> shape,wcs = a.shape,a.wcs
>>> oshape,owcs = enmap.slice_geometry(shape,wcs,np.s_[0,10:,10:])
>>> oshape
(3, 90, 90)
```

We expect
```
(90, 90) 
```